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probability question

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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by baresi » Thu Jul 14, 2011 12:27 am
Please help me understand how to calculate the answer to this question from Beat The Gmat Practice.

John has 4 girl friends and 5 boy friends. How many different ways can he invite 2 boys and 2 girls to his birthday?

Answer is 60, but how do you work it out? I thought it would be the 9!/5!+4! method but clearly not. Very frustrating!
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by Frankenstein » Thu Jul 14, 2011 1:14 am
Hi,
From the 4 girls, he can invite 2 in 4C2 = 6 ways
From the 5 boys, he can invite 2 in 5C2 = 10 ways
So, total number of ways is 6*10 = 60.
Cheers!

Things are not what they appear to be... nor are they otherwise
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by baresi » Thu Jul 14, 2011 2:18 am
Excuse my ignorance but what function does C perform in the above?
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by Frankenstein » Thu Jul 14, 2011 2:41 am
baresi wrote:Excuse my ignorance but what function does C perform in the above?
Hi,
C here denotes Combination. May be you have studied from sources in which it is represented as C(n,r)
where C(n,r) = n!/r!(n-r)!
So, in this question 4C2 can be denoted by C(4,2) = 4!/2!*2!.
Similarly, 5C2 can be denoted by C(5,2) = 5!/2!*3!
Cheers!

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