#1 permutation problem

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#1 permutation problem

by nailyad » Wed Dec 16, 2009 1:45 pm
Of the 12 temp.empoyee 4 will be hired as permanent. If 5 of the 12 temp.are woman, how many of the possible groups of 4 temp.employees consist of 3 woman and 1 man?

Answer please ?

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by Brent@GMATPrepNow » Wed Dec 16, 2009 4:44 pm
nailyad wrote:Of the 12 temp.empoyee 4 will be hired as permanent. If 5 of the 12 temp.are woman, how many of the possible groups of 4 temp.employees consist of 3 woman and 1 man?

Answer please ?
Divide this counting question into two stages:
Stage 1: Select 3 women from the 5 women
Stage 2: Select 1 man from the 7 men

Stage 1: Important question: does order in which we select the 3 women matter? For example, is selecting Ann, Bea and Dawn different from selecting Bea, Ann and Dawn? The answer is no, so we can use combinations.
We can accomplish stage 1 is 5C3 ways (10 ways)

Stage 2: We can select 1 man from 7 men in 7 ways.

The total number of ways to accomplish stage 1 and stage 2 is 10 x 7 = 70
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by viju9162 » Wed Dec 16, 2009 10:05 pm
Hi Brent,

Thanks for the explanation. I arrived until the answer (70), but I again divide it by 12C4. I believe that this approach should be done for determining the total number of possibilities when it asks for probability right?

Can you please guide here.

Thank you,
Viju
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by Testluv » Wed Dec 16, 2009 10:50 pm
viju9162 wrote:Hi Brent,

Thanks for the explanation. I arrived until the answer (70), but I again divide it by 12C4. I believe that this approach should be done for determining the total number of possibilities when it asks for probability right?

Can you please guide here.

Thank you,
Viju
That's correct. Had this been a probability question, we could use the formula:

Probability of desired outcome = Number of desired outcomes/number of total possible outcomes

in which 12C4 (the number of ways of selecting unordered subgroups of 4 from big group of 12) would be the denominator and 70 the numerator.

But in this question we don't care about the number of possible groups; instead, we just care about "how many of them" have 3 women and 1 man.

So, the key was the wording of the question. Because it is asking "how many" and because we are dealing with indivisible objects (people), we need a positive integer--not a proportion or fraction--in order to answer this question. If the question had asked for probability or fraction, then that would have been the correct aproach.
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by viju9162 » Wed Dec 16, 2009 10:55 pm
Thanks Testluv for the explanation :).

I got the difference now.

Regards,
Viju
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