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-   -   Simple Question (http://archives2.twoplustwo.com/showthread.php?t=354500)

closer2313 10-10-2005 01:18 PM

Simple Question
 
For some reason I can't get the right answer for this.

Suppose a vase has 15 balls, of which 8 are red and seven are black.

In how many ways can 5 balls be chosen so that at least two are red?

Tom1975 10-10-2005 02:29 PM

Re: Simple Question
 
[ QUOTE ]
For some reason I can't get the right answer for this.

Suppose a vase has 15 balls, of which 8 are red and seven are black.

In how many ways can 5 balls be chosen so that at least two are red?

[/ QUOTE ]

C(8,2)*C(7,3)+C(8,3)*C(7,2)+C(8,4)*C(7,1)+C(8,5)

This assumes it doesn't matter which order the balls are chosen.

10-10-2005 02:34 PM

Re: Simple Question
 
This question is insufficiently clear.

Do you mean:
8 chose 2 * 7 chose 3 +
8 chose 3 * 7 chose 2 +
8 chose 4 * 7 chose 1 +
8 chose 5 * 7 chose 0 =
2702
?

BruceZ 10-10-2005 03:21 PM

Re: Simple Question
 
[ QUOTE ]
For some reason I can't get the right answer for this.

Suppose a vase has 15 balls, of which 8 are red and seven are black.

In how many ways can 5 balls be chosen so that at least two are red?

[/ QUOTE ]

C(15,5) - C(7,5) - 8*C(7,4) = 2702.

closer2313 10-10-2005 03:24 PM

Re: Simple Question
 
Thanks alot, I forgot to subtract out all combinations of 5 black.


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