People use the word 'trick' to mean a variety of things. Sometimes people mean test-taking 'tricks' like 'backsolving' or 'picking numbers'. Sometimes people mean mathematical 'tricks', like using units digits or divisibility properties to pick a right answer without doing any real calculation. And sometimes people mean memorizing formulas for specific obscure scenarios.
The emphasis you should place on test-taking 'tricks' really depends on your level. Techniques like backsolving can be very successful on lower level questions, but are usually impossible to use on high-level questions. Even when they can be used, they're also almost always (with a few exceptions) more time-consuming than other approaches on harder questions.
I also don't advise memorizing a lot of inessential formulas, especially if you do that as a substitute for genuinely understanding the math. You will have so few opportunities to use formulas other than the standard ones that it's not worthwhile devoting time to memorizing them - that time will be better spent understanding concepts, and learning how to solve different types of questions.
Genuine mathematical tricks can be very useful, though. I find on any given GMAT test, I have at least a dozen opportunities to use some kind of time-saving trick. But those tricks come down to understanding how math works, and recognizing opportunities to apply that understanding. I'll give just one of many examples. Suppose you saw this question:
Keaton Cinemas operates 187 movie theaters, each containing 96 seats. How many seats are there in all of the Keaton Cinema theaters combined?
A) 17,952
B) 18,064
C) 18,128
D) 18,397
E) 18,796
This is not a GMAT-like problem, because it's too boring, but if I saw something similar on a GMAT, I'd know there must be a way to bypass the tedious computation of 187*96. Estimation does not look promising, but the answers all end in a different digit. Since the units digit of 187*96 must be 2, the only possible answer is A.
I can use a real GMAT problem to illustrate how you can use the same 'trick' to get the answer in three seconds - this is an old official problem:
If it is 5:27 in the morning now, what time in the evening was it 2,880,717 minutes ago?
A) 5:21
B) 5:24
C) 5:27
D) 5:30
E) 5:35
Many test takers would spend a long time on this question dividing up the 2,880,717 into hours and minutes. But we're really just subtracting two things ending in 7 - if you go back 7 minutes, or 17 minutes, or 27 minutes, or 2,880,717 minutes from 5:27, the time you get will always end in 0. So the answer must be 5:30.
That's one of dozens of tricks you can use to save time on the GMAT, and those are the tricks I find most useful. They're all based on having a good understanding of simple mathematical properties.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com
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