GMAT Quant

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GMAT Quant

by Suman_sourav » Tue Aug 28, 2012 12:36 am
A committee of 6 is chosen from 8 men and 5 women so as to contain at least 2 men and 3 women. How many different committees could be formed if two of the men refuse to serve together?
(A) 3510
(B) 2620
(C) 1404
(D) 700
(E) 635

This was a question that I got from another forum, however the way i solved it and the solutions that were posted in the forum did not match.

My solution:-
Since we must include at least 2 men and 3 women, we have just one variable, i.e, the 6th member. So, the sixth one can be both a man or a woman. hence the number of ways are-

1)6th man-(3 men and 3 women)
8C3 * 5C3 = 560

2) 6Th woman-(2 men and 4 women)
8C2 * 5C4 = 140
Combining these two, the answer comes to be 700.

However,the answer that I got in the forum is the following-

Committee can have either: 2 men and 4 women OR 3 men and 3 women (to meet the condition of at least 2 men and 3 women).

Ways to chose 6 members committee without restriction (two men refuse to server together):

Ways to chose 6 members committee with two particular men serve together:

700-65 = 635

Answer: E.

I would love some clarification as I did not understand why my method yields the wrong answer. Also, I am not quite fond of the way the problem was solved in the forum.

Any thoughts would be helpful.
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by vk_vinayak » Tue Aug 28, 2012 1:00 am
1)6th man-(3 men and 3 women)
8C3 * 5C3 = 560

2) 6Th woman-(2 men and 4 women)
8C2 * 5C4 = 140
Combining these two, the answer comes to be 700.

You are ignoring the restriction in both the cases:

case1: 3 men, 3 women:

(8C3 - 6)*(5C3) = 500. Here we need to subtract by 6 because there would be 6 groups which contain a particular pair of men who do not want to be together.

Case : 2 men, 4 women:

(8C2 - 1)* (5C4) = 135. Here as we are choosing 2 men, there would be 1 group in which those two men are together

Ans: 500 + 135 = 635

But the solution you got from a forum looks to easier and efficient method.
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by mehaksal » Tue Aug 28, 2012 2:22 am
vk_vinayak can u pls elaborate on how there is -6 in case1 and -1 in case2??

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by vk_vinayak » Tue Aug 28, 2012 2:36 am
mehaksal wrote:vk_vinayak can u pls elaborate on how there is -6 in case1 and -1 in case2??
The restriction in the question: two of the men refuse to serve together.

Let men be: A B C D E F G H

Suppose B and D refuse to serve together ie if we put B in a group, we shouldn't put D, and vice versa. They both can't appear in the same group.

Case1: 3 men and 3 women.

We need to decide how many groups of 3 men are there in which both B and D are together. If you put B and D together, the third person of that group can be included in 6 ways. ABD, CBD, EBD, FBD, GBD, HBD. Thus we need to subtract 6.

Case2: 2 men 3 women

We need to decide how many groups consisting of 2 men are there in which both B and D are together. Obviously there is only one way possible in which they are together ie BD. So we need to subtract one.

Hope it helps!
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by Suman_sourav » Tue Aug 28, 2012 8:07 am
Thanks a lot vk_vinayak. I realized after your post that the restriction actually matters. I initially thought that that restriction was immaterial.

Thanks for your way of solving it too. It shows your ingenuity.

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by vk_vinayak » Tue Aug 28, 2012 8:35 am
Suman_sourav wrote: I realized after your post that the restriction actually matters. I initially thought that that restriction was immaterial.
Restrictions are what make questions trickier. Remember, GMAT will not include anything extra in the question i.e. every word is important in solving the question.
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