Re: A firm has at least 5 partners, of whom at least 2 are male and at
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13 Nov 2017, 01:09
Here's a KEY that uses EXHAUSTIVE LISTS. Even if you solved this problem with combinatorial formulas, you should ALSO practice MAKING EXHAUSTIVE LISTS when you REVIEW it!
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Statement (1)
Together with the prompt, this statement tells us that the firm has either 2 or 3 male partners. There is no maximum, however, on the number of female partners; we know only that there are at least 2 female partners (and there can’t be just 2 of each, since the firm has at least 5 partners).
Let’s test cases:
• If there are 3 female partners (call them ‘1’, ‘2’, ‘3’) and 2 male partners (‘A’, ‘B’), then there are five ways to select 4 of them all. The selections consisting of 2 women and 2 men are boldfaced and asterisked:
1, 2, 3, A
1, 2, 3, B
1, 2, A, B*
1, 3, A, B*
2, 3, A, B*
Exactly three of these selections (the last three listed) are made up of 2 women and 2 men. So in this case the desired probability is 3/5.
• If there are 4 female partners (‘1’, ‘2’, ‘3’, ‘4’) and 2 male partners (‘A’, ‘B’), then there are 15 ways to select four of the six. The selections consisting of 2 women and 2 men are boldfaced and asterisked:
1, 2, 3, 4
1, 2, 3, A
1, 2, 3, B
1, 2, 4, A
1, 2, 4, B
1, 2, A, B*
1, 3, 4, A
1, 3, 4, B
1, 3, A, B*
1, 4, A, B*
2, 3, 4, A
2, 3, 4, B
2, 3, A, B*
2, 4, A, B*
3, 4, A, B*
Of the 15 possible selections, exactly 6 contain equal numbers of women and men (two of each). The desired probability in this case is thus 6/15 = 2/5.
These two cases produce two different answers to the problem, so, Statement (1) is NOT SUFFICIENT.
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Statement (2)
This time, we know that there are either 2 or 3 female partners, while the number of male partners can be any integer from 2 onward.
These are precisely the same numerical restrictions as in Statement (1), except with men and women reversed.
This statement is thus functionally identical to Statement (1) and is therefore NOT SUFFICIENT as well.
(Just switch men and women in the workup above. With 3 male and 2 female partners, the probability is 3/5; with 4 male and 2 female partners, the probability is 2/5.)
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Statements (1) and (2) TOGETHER
With both statements in force, there are only three possibilities for the partners overall:
• 3 women and 2 men
• 2 women and 3 men
• 3 women and 3 men
In either of the first two cases — as demonstrated under Statement (1) — the desired probability is 3/5.
The only case left to consider is that involving 3 women (‘1’, ‘2’, ‘3’) and 3 men (‘A’, ‘B’, ‘C’). There are, again, 15 ways to choose four of the six.
(Note that you don’t have to do this work again from scratch! You can simply copy the list made above for 4 women and 2 men, substituting ‘C’ for ‘4’.)
1, 2, 3, C
1, 2, 3, A
1, 2, 3, B
1, 2, C, A*
1, 2, C, B*
1, 2, A, B*
1, 3, C, A*
1, 3, C, B*
1, 3, A, B*
1, C, A, B
2, 3, C, A*
2, 3, C, B*
2, 3, A, B*
2, C, A, B
3, C, A, B
Of the 15 possible selections, exactly 9 contain equal numbers of women and men (two of each). The desired probability in this case is thus 9/15 = 3/5.
The probability is thus 3/5 for all cases possible under both statements. The two statements TOGETHER are thus SUFFICIENT.
The correct answer is C.