GMAT Tips – Sample Problem: Data Sufficiency Combinations
Today’s GMAT Tip comes to us from Kaplan. In this article, Kaplan GMAT instructor Bret Ruber explains how to tackle a Data Sufficiency problem involving combinations:
Sometimes the challenge of specific GMAT problems is that they combine a higher-level concept such as Combinations, with a Data Sufficiency question, with some algebra thrown in as well. But once you know the basics of dealing with Data Sufficiency, and the formula and concepts of Combinations, you can just follow a step-by-step approach to a problem such as this:
Sample Problem:
Integers x and y are both positive, and x > y. How many different committees of y people can be chosen from a group of x people?
(1) The number of different committees of x-y people that can be chosen from a group of x people is 3,060.
(2) The number of different ways to arrange x-y people in a line is 24.
Solution:
The first step in this problem is to determine what we are really being asked. If we want to select committees of y people from a group of x people, we should use the combinations formula, which is n!/[k!/(n-k)!]. Remember, in this formula n is the number with which we start and k is the number we want in each group. Thus, we can reword the question as what does x!/[y!(x-y)!] equal?
Statement 1 tells us how many committees of x-y people we can make from our initial group of x people. If we plug this information into the combinations formula, we get x!/[(x-y)!(x-(x-y))!] = 3,060. This can be simplified to x!/[(x-y)!(x-x+y))!] = 3,060, which in turn is simplified to x!/[(x-y)!y!] = 3,060. The simplified equation matches the expression in our question, and gives us a numerical solution for it. Therefore, statement 1 is sufficient.
Statement 2 tells us how many ways we can arrange a number of people. The formula for arrangements is simply n!. In this case we have x-y people, thus (x-y)! = 24. Therefore, x-y must equal 4. However, we have no way of calculating what x and y actually are. This means that we cannot calculate the number of combinations in our question. Statement 2 is insufficient. So our final answer choice for this Data Sufficiency question is answer choice (A) or (1), Statement 1 is sufficient on its own, but Statement 2 is not.
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