probability - selecting balls from a box
$begingroup$
A box contains 4 white, 5 red, and 6 black balls, four balls are drawn at random from the box. Then the probability that among the balls drawn there is at least 1 ball of each color is?
What goes wrong in my approach:
$$frac{4C1 cdot 5C1 cdot 6C1 cdot 12}{15C4}$$
probability combinatorics combinations
$endgroup$
|
show 4 more comments
$begingroup$
A box contains 4 white, 5 red, and 6 black balls, four balls are drawn at random from the box. Then the probability that among the balls drawn there is at least 1 ball of each color is?
What goes wrong in my approach:
$$frac{4C1 cdot 5C1 cdot 6C1 cdot 12}{15C4}$$
probability combinatorics combinations
$endgroup$
2
$begingroup$
You are overcounting. You count the choice $(W_1,R_1,B_1,W_2)$ and $(W_2,R_1,B_1,W_1)$ as two distinct choices.
$endgroup$
– lulu
Dec 17 '18 at 15:09
1
$begingroup$
oh got it. Thanks I should have done selecting 2 balls from each color followed by other 2 colors one ball each.
$endgroup$
– swapnil
Dec 17 '18 at 15:13
1
$begingroup$
As an alternative: you can argue that your method exactly double counts. That way, all you need to do is to divide your answer by $2$.
$endgroup$
– lulu
Dec 17 '18 at 15:19
$begingroup$
I would do it like that : $p = frac{3 cdot 4!}{15C4}$ where the nominator describe the permutations of 4 balls, 3 of different colors and the last one is either red or white or black
$endgroup$
– Alain.Klbtr
Dec 17 '18 at 15:32
$begingroup$
This tutorial explains how to typeset mathematics on this site. Also, when you pose an attempt here, you should explain how you got your answer since that helps users identify any errors you may have made.
$endgroup$
– N. F. Taussig
Dec 17 '18 at 15:32
|
show 4 more comments
$begingroup$
A box contains 4 white, 5 red, and 6 black balls, four balls are drawn at random from the box. Then the probability that among the balls drawn there is at least 1 ball of each color is?
What goes wrong in my approach:
$$frac{4C1 cdot 5C1 cdot 6C1 cdot 12}{15C4}$$
probability combinatorics combinations
$endgroup$
A box contains 4 white, 5 red, and 6 black balls, four balls are drawn at random from the box. Then the probability that among the balls drawn there is at least 1 ball of each color is?
What goes wrong in my approach:
$$frac{4C1 cdot 5C1 cdot 6C1 cdot 12}{15C4}$$
probability combinatorics combinations
probability combinatorics combinations
edited Dec 17 '18 at 16:18
N. F. Taussig
44.3k93357
44.3k93357
asked Dec 17 '18 at 15:06
swapnilswapnil
335
335
2
$begingroup$
You are overcounting. You count the choice $(W_1,R_1,B_1,W_2)$ and $(W_2,R_1,B_1,W_1)$ as two distinct choices.
$endgroup$
– lulu
Dec 17 '18 at 15:09
1
$begingroup$
oh got it. Thanks I should have done selecting 2 balls from each color followed by other 2 colors one ball each.
$endgroup$
– swapnil
Dec 17 '18 at 15:13
1
$begingroup$
As an alternative: you can argue that your method exactly double counts. That way, all you need to do is to divide your answer by $2$.
$endgroup$
– lulu
Dec 17 '18 at 15:19
$begingroup$
I would do it like that : $p = frac{3 cdot 4!}{15C4}$ where the nominator describe the permutations of 4 balls, 3 of different colors and the last one is either red or white or black
$endgroup$
– Alain.Klbtr
Dec 17 '18 at 15:32
$begingroup$
This tutorial explains how to typeset mathematics on this site. Also, when you pose an attempt here, you should explain how you got your answer since that helps users identify any errors you may have made.
$endgroup$
– N. F. Taussig
Dec 17 '18 at 15:32
|
show 4 more comments
2
$begingroup$
You are overcounting. You count the choice $(W_1,R_1,B_1,W_2)$ and $(W_2,R_1,B_1,W_1)$ as two distinct choices.
$endgroup$
– lulu
Dec 17 '18 at 15:09
1
$begingroup$
oh got it. Thanks I should have done selecting 2 balls from each color followed by other 2 colors one ball each.
$endgroup$
– swapnil
Dec 17 '18 at 15:13
1
$begingroup$
As an alternative: you can argue that your method exactly double counts. That way, all you need to do is to divide your answer by $2$.
$endgroup$
– lulu
Dec 17 '18 at 15:19
$begingroup$
I would do it like that : $p = frac{3 cdot 4!}{15C4}$ where the nominator describe the permutations of 4 balls, 3 of different colors and the last one is either red or white or black
$endgroup$
– Alain.Klbtr
Dec 17 '18 at 15:32
$begingroup$
This tutorial explains how to typeset mathematics on this site. Also, when you pose an attempt here, you should explain how you got your answer since that helps users identify any errors you may have made.
$endgroup$
– N. F. Taussig
Dec 17 '18 at 15:32
2
2
$begingroup$
You are overcounting. You count the choice $(W_1,R_1,B_1,W_2)$ and $(W_2,R_1,B_1,W_1)$ as two distinct choices.
$endgroup$
– lulu
Dec 17 '18 at 15:09
$begingroup$
You are overcounting. You count the choice $(W_1,R_1,B_1,W_2)$ and $(W_2,R_1,B_1,W_1)$ as two distinct choices.
$endgroup$
– lulu
Dec 17 '18 at 15:09
1
1
$begingroup$
oh got it. Thanks I should have done selecting 2 balls from each color followed by other 2 colors one ball each.
$endgroup$
– swapnil
Dec 17 '18 at 15:13
$begingroup$
oh got it. Thanks I should have done selecting 2 balls from each color followed by other 2 colors one ball each.
$endgroup$
– swapnil
Dec 17 '18 at 15:13
1
1
$begingroup$
As an alternative: you can argue that your method exactly double counts. That way, all you need to do is to divide your answer by $2$.
$endgroup$
– lulu
Dec 17 '18 at 15:19
$begingroup$
As an alternative: you can argue that your method exactly double counts. That way, all you need to do is to divide your answer by $2$.
$endgroup$
– lulu
Dec 17 '18 at 15:19
$begingroup$
I would do it like that : $p = frac{3 cdot 4!}{15C4}$ where the nominator describe the permutations of 4 balls, 3 of different colors and the last one is either red or white or black
$endgroup$
– Alain.Klbtr
Dec 17 '18 at 15:32
$begingroup$
I would do it like that : $p = frac{3 cdot 4!}{15C4}$ where the nominator describe the permutations of 4 balls, 3 of different colors and the last one is either red or white or black
$endgroup$
– Alain.Klbtr
Dec 17 '18 at 15:32
$begingroup$
This tutorial explains how to typeset mathematics on this site. Also, when you pose an attempt here, you should explain how you got your answer since that helps users identify any errors you may have made.
$endgroup$
– N. F. Taussig
Dec 17 '18 at 15:32
$begingroup$
This tutorial explains how to typeset mathematics on this site. Also, when you pose an attempt here, you should explain how you got your answer since that helps users identify any errors you may have made.
$endgroup$
– N. F. Taussig
Dec 17 '18 at 15:32
|
show 4 more comments
1 Answer
1
active
oldest
votes
$begingroup$
Your method counts every favorable selection twice, once for each way you could designate one of the balls from the color that is selected twice as the ball of that color.
Let's compare your approach with the correct solution.
Solution: The number of ways we may select four of the $4 + 5 + 6 = 15$ balls is
$$binom{15}{4}$$
If at least one ball of each color is selected, then since four balls are being selected from three colors, exactly one color must be selected twice with the other two colors each selected once.
Two white balls, one red ball, and one black ball are selected: We must select two of the four white balls, one of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{2}binom{5}{1}binom{6}{1}$$
ways.
One white ball, two red balls, and one black ball are selected: We must select one of the four white balls, two of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{1}binom{5}{2}binom{6}{1}$$
ways.
One white ball, one red ball, and two black balls are selected: We must select one of the four white balls, one of the five red balls, and two of the six black balls, which can be done in
$$binom{4}{1}binom{5}{1}binom{6}{2}$$
ways.
Number of favorable cases: Since the three cases above are mutually exclusive and exhaustive, the number of favorable cases is found by adding the number of selections for each case.
$$binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}binom{6}{2}$$
Probability: Assuming each ball is selected randomly, the probability of selecting at least one ball of each color is
$$frac{dbinom{4}{2}dbinom{5}{1}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{2}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{1}dbinom{6}{2}}{dbinom{15}{4}}$$
What error did you make?
By designating a ball of each color as the ball reserved for that color, you count each favorable selection twice, once for each way you could have designated one of those balls as the ball of that color. To borrow @lulu's example, you count the selection $W_1, W_2, R_1, B_1$ twice:
begin{array}{c c}
text{designated balls} & text{additional ball}\ hline
W_1, R_1, B_1 & W_2\
W_2, R_1, B_1 & W_1
end{array}
Notice that
$$color{red}{binom{2}{1}}binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}color{red}{binom{2}{1}}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}color{red}{binom{2}{1}}binom{6}{2} = binom{4}{1}binom{5}{1}binom{6}{1} cdot 12$$
where the factor $color{red}{binom{2}{1}}$ represents the number of ways you could have designated one of the two balls from the color that is selected twice as the representative ball of that color.
$endgroup$
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3044060%2fprobability-selecting-balls-from-a-box%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Your method counts every favorable selection twice, once for each way you could designate one of the balls from the color that is selected twice as the ball of that color.
Let's compare your approach with the correct solution.
Solution: The number of ways we may select four of the $4 + 5 + 6 = 15$ balls is
$$binom{15}{4}$$
If at least one ball of each color is selected, then since four balls are being selected from three colors, exactly one color must be selected twice with the other two colors each selected once.
Two white balls, one red ball, and one black ball are selected: We must select two of the four white balls, one of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{2}binom{5}{1}binom{6}{1}$$
ways.
One white ball, two red balls, and one black ball are selected: We must select one of the four white balls, two of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{1}binom{5}{2}binom{6}{1}$$
ways.
One white ball, one red ball, and two black balls are selected: We must select one of the four white balls, one of the five red balls, and two of the six black balls, which can be done in
$$binom{4}{1}binom{5}{1}binom{6}{2}$$
ways.
Number of favorable cases: Since the three cases above are mutually exclusive and exhaustive, the number of favorable cases is found by adding the number of selections for each case.
$$binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}binom{6}{2}$$
Probability: Assuming each ball is selected randomly, the probability of selecting at least one ball of each color is
$$frac{dbinom{4}{2}dbinom{5}{1}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{2}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{1}dbinom{6}{2}}{dbinom{15}{4}}$$
What error did you make?
By designating a ball of each color as the ball reserved for that color, you count each favorable selection twice, once for each way you could have designated one of those balls as the ball of that color. To borrow @lulu's example, you count the selection $W_1, W_2, R_1, B_1$ twice:
begin{array}{c c}
text{designated balls} & text{additional ball}\ hline
W_1, R_1, B_1 & W_2\
W_2, R_1, B_1 & W_1
end{array}
Notice that
$$color{red}{binom{2}{1}}binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}color{red}{binom{2}{1}}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}color{red}{binom{2}{1}}binom{6}{2} = binom{4}{1}binom{5}{1}binom{6}{1} cdot 12$$
where the factor $color{red}{binom{2}{1}}$ represents the number of ways you could have designated one of the two balls from the color that is selected twice as the representative ball of that color.
$endgroup$
add a comment |
$begingroup$
Your method counts every favorable selection twice, once for each way you could designate one of the balls from the color that is selected twice as the ball of that color.
Let's compare your approach with the correct solution.
Solution: The number of ways we may select four of the $4 + 5 + 6 = 15$ balls is
$$binom{15}{4}$$
If at least one ball of each color is selected, then since four balls are being selected from three colors, exactly one color must be selected twice with the other two colors each selected once.
Two white balls, one red ball, and one black ball are selected: We must select two of the four white balls, one of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{2}binom{5}{1}binom{6}{1}$$
ways.
One white ball, two red balls, and one black ball are selected: We must select one of the four white balls, two of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{1}binom{5}{2}binom{6}{1}$$
ways.
One white ball, one red ball, and two black balls are selected: We must select one of the four white balls, one of the five red balls, and two of the six black balls, which can be done in
$$binom{4}{1}binom{5}{1}binom{6}{2}$$
ways.
Number of favorable cases: Since the three cases above are mutually exclusive and exhaustive, the number of favorable cases is found by adding the number of selections for each case.
$$binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}binom{6}{2}$$
Probability: Assuming each ball is selected randomly, the probability of selecting at least one ball of each color is
$$frac{dbinom{4}{2}dbinom{5}{1}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{2}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{1}dbinom{6}{2}}{dbinom{15}{4}}$$
What error did you make?
By designating a ball of each color as the ball reserved for that color, you count each favorable selection twice, once for each way you could have designated one of those balls as the ball of that color. To borrow @lulu's example, you count the selection $W_1, W_2, R_1, B_1$ twice:
begin{array}{c c}
text{designated balls} & text{additional ball}\ hline
W_1, R_1, B_1 & W_2\
W_2, R_1, B_1 & W_1
end{array}
Notice that
$$color{red}{binom{2}{1}}binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}color{red}{binom{2}{1}}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}color{red}{binom{2}{1}}binom{6}{2} = binom{4}{1}binom{5}{1}binom{6}{1} cdot 12$$
where the factor $color{red}{binom{2}{1}}$ represents the number of ways you could have designated one of the two balls from the color that is selected twice as the representative ball of that color.
$endgroup$
add a comment |
$begingroup$
Your method counts every favorable selection twice, once for each way you could designate one of the balls from the color that is selected twice as the ball of that color.
Let's compare your approach with the correct solution.
Solution: The number of ways we may select four of the $4 + 5 + 6 = 15$ balls is
$$binom{15}{4}$$
If at least one ball of each color is selected, then since four balls are being selected from three colors, exactly one color must be selected twice with the other two colors each selected once.
Two white balls, one red ball, and one black ball are selected: We must select two of the four white balls, one of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{2}binom{5}{1}binom{6}{1}$$
ways.
One white ball, two red balls, and one black ball are selected: We must select one of the four white balls, two of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{1}binom{5}{2}binom{6}{1}$$
ways.
One white ball, one red ball, and two black balls are selected: We must select one of the four white balls, one of the five red balls, and two of the six black balls, which can be done in
$$binom{4}{1}binom{5}{1}binom{6}{2}$$
ways.
Number of favorable cases: Since the three cases above are mutually exclusive and exhaustive, the number of favorable cases is found by adding the number of selections for each case.
$$binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}binom{6}{2}$$
Probability: Assuming each ball is selected randomly, the probability of selecting at least one ball of each color is
$$frac{dbinom{4}{2}dbinom{5}{1}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{2}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{1}dbinom{6}{2}}{dbinom{15}{4}}$$
What error did you make?
By designating a ball of each color as the ball reserved for that color, you count each favorable selection twice, once for each way you could have designated one of those balls as the ball of that color. To borrow @lulu's example, you count the selection $W_1, W_2, R_1, B_1$ twice:
begin{array}{c c}
text{designated balls} & text{additional ball}\ hline
W_1, R_1, B_1 & W_2\
W_2, R_1, B_1 & W_1
end{array}
Notice that
$$color{red}{binom{2}{1}}binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}color{red}{binom{2}{1}}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}color{red}{binom{2}{1}}binom{6}{2} = binom{4}{1}binom{5}{1}binom{6}{1} cdot 12$$
where the factor $color{red}{binom{2}{1}}$ represents the number of ways you could have designated one of the two balls from the color that is selected twice as the representative ball of that color.
$endgroup$
Your method counts every favorable selection twice, once for each way you could designate one of the balls from the color that is selected twice as the ball of that color.
Let's compare your approach with the correct solution.
Solution: The number of ways we may select four of the $4 + 5 + 6 = 15$ balls is
$$binom{15}{4}$$
If at least one ball of each color is selected, then since four balls are being selected from three colors, exactly one color must be selected twice with the other two colors each selected once.
Two white balls, one red ball, and one black ball are selected: We must select two of the four white balls, one of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{2}binom{5}{1}binom{6}{1}$$
ways.
One white ball, two red balls, and one black ball are selected: We must select one of the four white balls, two of the five red balls, and one of the six black balls, which can be done in
$$binom{4}{1}binom{5}{2}binom{6}{1}$$
ways.
One white ball, one red ball, and two black balls are selected: We must select one of the four white balls, one of the five red balls, and two of the six black balls, which can be done in
$$binom{4}{1}binom{5}{1}binom{6}{2}$$
ways.
Number of favorable cases: Since the three cases above are mutually exclusive and exhaustive, the number of favorable cases is found by adding the number of selections for each case.
$$binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}binom{6}{2}$$
Probability: Assuming each ball is selected randomly, the probability of selecting at least one ball of each color is
$$frac{dbinom{4}{2}dbinom{5}{1}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{2}dbinom{6}{1} + dbinom{4}{1}dbinom{5}{1}dbinom{6}{2}}{dbinom{15}{4}}$$
What error did you make?
By designating a ball of each color as the ball reserved for that color, you count each favorable selection twice, once for each way you could have designated one of those balls as the ball of that color. To borrow @lulu's example, you count the selection $W_1, W_2, R_1, B_1$ twice:
begin{array}{c c}
text{designated balls} & text{additional ball}\ hline
W_1, R_1, B_1 & W_2\
W_2, R_1, B_1 & W_1
end{array}
Notice that
$$color{red}{binom{2}{1}}binom{4}{2}binom{5}{1}binom{6}{1} + binom{4}{1}color{red}{binom{2}{1}}binom{5}{2}binom{6}{1} + binom{4}{1}binom{5}{1}color{red}{binom{2}{1}}binom{6}{2} = binom{4}{1}binom{5}{1}binom{6}{1} cdot 12$$
where the factor $color{red}{binom{2}{1}}$ represents the number of ways you could have designated one of the two balls from the color that is selected twice as the representative ball of that color.
answered Dec 18 '18 at 11:08
N. F. TaussigN. F. Taussig
44.3k93357
44.3k93357
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3044060%2fprobability-selecting-balls-from-a-box%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
2
$begingroup$
You are overcounting. You count the choice $(W_1,R_1,B_1,W_2)$ and $(W_2,R_1,B_1,W_1)$ as two distinct choices.
$endgroup$
– lulu
Dec 17 '18 at 15:09
1
$begingroup$
oh got it. Thanks I should have done selecting 2 balls from each color followed by other 2 colors one ball each.
$endgroup$
– swapnil
Dec 17 '18 at 15:13
1
$begingroup$
As an alternative: you can argue that your method exactly double counts. That way, all you need to do is to divide your answer by $2$.
$endgroup$
– lulu
Dec 17 '18 at 15:19
$begingroup$
I would do it like that : $p = frac{3 cdot 4!}{15C4}$ where the nominator describe the permutations of 4 balls, 3 of different colors and the last one is either red or white or black
$endgroup$
– Alain.Klbtr
Dec 17 '18 at 15:32
$begingroup$
This tutorial explains how to typeset mathematics on this site. Also, when you pose an attempt here, you should explain how you got your answer since that helps users identify any errors you may have made.
$endgroup$
– N. F. Taussig
Dec 17 '18 at 15:32