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GMAT Tips: Multiplication Shortcuts on the GMAT [#permalink]
04 Jul 2011, 03:36
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When working on the GMAT quantitative section, it is always important to remember that the questions are written so that they can be completed within about a two-minute timeframe. If you encounter a problem and the math seems as if it will take more than two minutes to do, it generally means that either you made an error or a faster way to solve exists. One of the most frequent cases in which the latter occurs is on problems that involve multiplication, since there are no calculators on the GMAT.
Unlike long division, which can be very useful on the GMAT, longhand multiplication is almost never necessary. Instead you should always look for shortcuts to solve. Not only will this be quicker, but it will also provide fewer opportunities for careless errors.
One such shortcut is prime factorization. If a problem asks you to multiply 525 by 16, you can break down 525 to 3 x 5 x 5 x 7 and 16 to 2 x 2 x 2 x 2 and multiply these strings of primes together, giving you 3 x 5 x 5 x 7 x 2 x 2 x 2 x 2. Since the order in which you multiply does not matter, recombine the primes into easier to use numbers. Especially keep an eye out for 5′s and 2′s you can make equal 10. Here we can rewrite our problem as (5 x 2) x (5 x 2) x (3 x 7) x (2 x 2) = 10 x 10 x 21 x 4. Again, look for multiplication you can complete quickly. 100 x 84 = 8400.
Another shortcut is to take a step back and consider what the multiplication problem really means. 64 x 9, is one fewer 64 than 64 x 10. So we can solve 64 x 10 = 640, then subtract 64, which equals 576.
Whenever you can find a faster way to reach the answer you should take it. The key is to remember, the right way to do the problem is the way that gets you to the correct answer most quickly.