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# There are 15 students in a group. 10 students of this group

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Intern
Joined: 22 Oct 2007
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There are 15 students in a group. 10 students of this group [#permalink]  22 Oct 2007, 09:18
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There are 15 students in a group. 10 students of this group are boys.
5 students were invited to an interview. What is a probability that 3 of them will be girls?
Director
Joined: 11 Jun 2007
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Re: PS Probability [#permalink]  22 Oct 2007, 09:59
Summerfish wrote:
There are 15 students in a group. 10 students of this group are boys.
5 students were invited to an interview. What is a probability that 3 of them will be girls?

prob that 3 will be girls from group of 5 = 5C3 =
5*4*3*2*1 / 3*2*1 = 20

total probability = 15C5 = 15! / 5! 10!
15*14*13*12*11 / 5*4*3*2*1 = 7*13*3*11 = 3003

so 20/3003
Senior Manager
Joined: 04 Jan 2006
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Re: PS Probability [#permalink]  22 Oct 2007, 10:05
Summerfish wrote:
There are 15 students in a group. 10 students of this group are boys.
5 students were invited to an interview. What is a probability that 3 of them will be girls?

Method (I)

10 Boys 5 Girls

Prob( 3 Girls and 2 Boys) = (5C3)x(5/15)(4/14)(3/13)(10/12)(9/11)
= 150/1001

Method (II)

Prob(3 Girls and 2 Boys) = (5C3)x(10C2)/(15C5)
= 150/1001
Intern
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Re: PS Probability [#permalink]  22 Oct 2007, 19:35
So, what is the right answer - 150/1001 or 20/3003 ?

When I counted - I got probability 1/9.

But I think that it is more likely to be 150/1001.
Intern
Joined: 22 Oct 2007
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Re: PS Probability [#permalink]  23 Oct 2007, 05:27
Who can confirm correct answer?
Manager
Joined: 26 Sep 2007
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[#permalink]  24 Oct 2007, 11:02
150/1001

Total outcomes n(S) = 15C5 = 3003
3 girls will be selected which means 3 girls out of 5 and 2 boys out of 10 are selected,
so n(E) = 5C3.10C2 = 450

P = n(E)/N(S) = 450/3003 = 150/1001
[#permalink] 24 Oct 2007, 11:02
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# There are 15 students in a group. 10 students of this group

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