How should I prep for GMAT data sufficiency questions?
Today's post will focus on data sufficiency questions, which are arguably the most difficult problem type found on the GMAT. Unlike problem solving questions, data sufficiency problems feel unfamiliar and often lead test-takers to make simple mistakes.
Before we get to today's sample question, let's review the best approach for data sufficiency questions. Each problem will have a question and perhaps one or more additional facts or pieces of information, followed by two statements and the answer choices. When approaching these types of problems, you should determine what the question is asking and whether it is asking for a specific value (e.g., What is the distance between A and B?) or a yes/no answer (Is x odd?).
Now we move to the statements and the answer choices. Again, there will be two statements - each should be read and considered independently (more on that in a minute). As for the answer choices, there is the common format that all data sufficiency questions, listed below:
(A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
(B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
(C) BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
(D) EACH statement ALONE is sufficient.
(E) Statements (1) and (2) TOGETHER are not sufficient.
Okay, I've got the structure. Now how do we tackle the statements?
First, read statement 1 while covering up statement 2. If you determine that statement 1 is sufficient, you can eliminate answer choices B, C, and E. If statement 1 is not sufficient, you can eliminate answer choices A and D.
Now move on to statement 2. If statement 2 is sufficient, then the answer choice must be B (if statement 1 was not sufficient) or D (if statement 1 was sufficient). If statement 2 was not sufficient but statement 1 was, then A is the correct answer.
Otherwise, look at both statements together to determine whether the statements together are sufficient (answer choice C) or not sufficient (answer choice E).
I know, it sounds very complicated – but it's not too hard once you get the hang of it.
Let's try a GMAT problem!
Here is a practice GMAT problem:
A taxi company charges f cents for the first mile of the taxi ride and m cents for each additional mile. How much does the company charge for a 10-mile taxi ride?
(1) The company charges $0.90 for a 2-mile ride.
(2) The company charges $1.20 for a 4-mile ride.
I don't know – maybe the taxi meter will tell us! How do we start?
The question asks for a specific value – in this case, the charge for a 10-mile taxi ride. The information in the problem implies that the taxi fare is calculated using the following formula (with cents converted to dollars): Taxi charge in dollars = f/100 + (m/100)*(miles traveled – 1). In order to calculate the charge for a 10-mile taxi ride, we need to solve for the exact values of f and m.
Now, let’s look at the statements individually:
Statement (1) This tells us that (f + m)/100 must equal 0.90, but we cannot solve for the individual values of f and m, so we cannot calculate the 10-mile taxi ride charge. We can eliminate answer choices A and D.
Statement (2) This tells us that (f + 3m)/100 must equal 1.20, but again we do not know the specific values of f and m. We can eliminate answer choice B.
Maybe we can't solve this one.
Not so fast - answer choices C and E remain. Examining the statements together, we see that we can create two equations use them to solve for both variables:
(1) 0.90 = (f + m)/100
(2) 1.20 = (f + 3m)/100
With a bit of arithmetic manipulation, equation (1) can be rearranged to f = 90 – m, and then plugged into equation 2:
1.20 = ((90 – m) + 3m)/100
120 = 90 + 2m
30 = 2m
15 = m
Plugging this value of m back into equation (1), we can calculate that f = 75. At this point, we know that both statements together are sufficient to answer the question – remember, we just need to know that the statements are sufficient to answer the question, we do not actually have to find the exact value! However, to prove this point, we can solve for the charge of the 10-mile taxi ride: 0.75 + 0.15*9 = 2.10. The correct answer is C.
Can I use this method for other GMAT data sufficiency problems?
Yes! Through this sample problem, I hope you saw that the strategy for approaching data sufficiency questions is straightforward once you do it a few times. Following these steps will also help you to avoid simple mistakesm, such as selecting E to a Yes/No question when one or both statements are sufficient to answer "No" to the question.
Tune in later this month for the next installment of the Math Mechanic.
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