GMAT Data Sufficiency tip: flesh out the prompt

It is very important to tackle data sufficiency in a formulaic way.  There are too many options to keep them in your mind all at once, so its better to methodically eliminate things until you have only one option left. The first and most important step is to flesh out the prompt, so you can gather as much information as you can from it.  Additional info, which can make or break the question, is often hidden in the prompt.  Take a look at this problem:

The difference between Simon and Gary’s age is twice as much as the difference between Simon and Chris’s age.   If Simon is the oldest, what is the average (arithmetic mean) of all three?

(1) Gary is 27

(2) Chris is 31

If we just did this problem superficially, then we would start by putting the prompt into an equation, such that

S-G=2(S-C) which we can multiply out to get

S=2C-G

We then might conclude that we only need two of the numbers to get the average, because we know the relationship between the three, but let’s try substituting S in the average equation ((S+G+C)/3) with 2C-G.

(2C-G+G+C)/3=3C/3=C

The Gs actually end up canceling out in the equation, so that all we need to know to figure out the average is Chris’s age.   Thus, going through the options is very simple.  (I) tells us nothing, because we would still need Simon’s or Chris’s age, but (II) gives us all the information we need to solve. Had we not taken the time to plug the one equation into the other, we probably would have incorrectly assumed that the correct answer was that we needed both together to answer the question.  However, it is now clear that we only need (II).

After you multiply out the prompt and gather all the information from that, you must then turn to your two pieced of information.  I find the easiest way to do this is to go through them in order: (I) then (II) . That way you don’t get confused about what you’ve done.  However, some people find it easier to start with the easier of the two options, so find what works for you and stick to it. 

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Eliza Chute

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