GMAT Tip: You Can't Always Get What You Want

This tip on improving your GMAT score was provided by Brian Galvin at Veritas Prep.
A common frustration among GMAT examinees is that the questions they see on test day “don’t look like” the questions they practiced, even those from the Official Guide for GMAT Review and other official GMAC-produced study materials. They’ve memorized formulas and problem structures for rates, triangles, averages, Venn diagrams—you name it—but many of the questions don’t lend themselves to the recitation of their notes and flash cards. Which reveals a secret:
The GMAT isn’t a content-based exam that tests “what you know,” but rather a reasoning-based test that uses that content to see “how you think.” In other words—let’s use Mick Jagger’s—you can’t always get what you want (a question that closely replicates one you’ve practiced a dozen times) but if you try, you’ll find you get what you need to solve the problem.
Which essentially means this: If you go into the GMAT having memorized the D = RT rate/distance/time formula and nothing else, you’ll struggle on rate problems that are above-average in difficulty. But if you use what you know about rates to “take what the test gives you” and leverage that to solve problems, you’ll likely be able to find what you need to get to a correct answer. Consider this problem:
A factory has three types of machines—the XS, the LS, and the RT—and each type of machine works at its own constant rate. If six XS machines and 13 LS machines can produce 140 widgets in an hour, and if seven XS machines and 13 RT machines can produce 120 widgets in an hour, how many hours will it take for a group of one XS, one LS, and one RT working together to produce 100 widgets?
(A) 3
(B) 5
(C) 7
(D) 9
(E) 13
While this problem may not look like a typical rate problem, the math required to solve it isn’t all that difficult. It’s much more a reasoning problem that hinges on conceptual knowledge of rates. If you remember that rates are additive (if I can produce two units an hour and you can produce three, together we produce five units an hour) and notice the similarities in the numbers, you have a good shot at recognizing that:
13 (Rate LS) + 6 (Rate XS) = 140 widgets / 1 hour
13 (Rate RT) + 7 (Rate XS) = 120 widgets / 1 hour
If you could find a way to add the 6 XS in the first equation with the 7 XS in the second, you’d have 13 of each—which brings up an algebra rule that you can add together equations. So you can combine both equations to get:
13 (Rate LS) + 13 (Rate RT) + 13 (Rate XS) = 260 widgets / 1 hour
And since you want the combination of one of each machine, you can then divide everything by 13 to get:
Rate LS + Rate RT + Rate XS = 20 widgets / 1 hour
That means that if one of each machine can do 20 per hour, then it will take 5 hours to complete all 100, making the correct answer B. But more important is the takeaway—the fact that a problem involves rates doesn’t mean that it’s a “plug what you see into the rate equation and chug away” problem. Harder GMAT problems make you blend together a range of skills and require you to bring in some ingenuity and problem-solving to determine which skills those are.
Or to paraphrase Mick Jagger, you won’t always see the problem you want to see, but if you try sometimes you’ll find that it’s a problem you know how to solve, just using a different blend of skills than you’ve combined before