For many GMAT examinees, the realization that they cannot use a calculator on the GMAT quantitative section is cause for despair. For most of your high school career, calculators were a featured part of the math curriculum (what TI are we up to now?); nowadays you almost always have Microsoft Excel a click away to perform those calculations for you.
But remember: it’s not that YOU don’t get to use a calculator on the GMAT quant section. It’s that NO ONE gets to use a calculator. And that creates the opportunity for a competitive advantage. If you know that the GMAT doesn’t include “calculator problems” – the testmakers know that you don’t get a calculator, too, so they create questions that savvy examinees can find efficient ways to solve by hand (or head) or estimate – then you can use that to your advantage, looking for “clean” numbers to calculate and saving calculations until they’re truly necessary. As an example, consider the problem:
A certain box contains 14 apples and 23 oranges. How many oranges must be removed from the box so that 70 percent of the pieces of fruit in the box will be apples?
If you’re well-versed in “non-calculator” math, you should recognize a couple things as you scan the problem:
1) The 23 oranges represent a prime number. That’s an ugly number to calculate with in a non-calculator problem.
2) 70% is a very clean number, which reduces to 7/10. Numbers that end in 0 don’t tend to play well with or come from double-digit prime numbers, so in this problem you’ll need to “clean up” that 23.
3) The 14 apples are pretty nicely related to the 70%. 14*5 = 70, and 14 = 7 * 2, where 70% is 7/10. So in sum, the 14 is a pretty “clean” number you’re working with to find a relationship that includes that also “clean” 70%. And the 23 is ugly.
So if you wanted to plug in numbers here to see how many oranges should be removed, keep in mind that your job is to get that 23 to look a lot cleaner. So while the Goldilocksian conventional methodology for backsolving is to “start with the middle number, then determine whether it’s correct, too big, or too small,” if you’re preparing for non-calculator math you should quickly see that with answer choice C of 14, that would give you 14 apples and 9 (which is 23-14) oranges), and you’re stuck at that ugly number of 23 as your total number of pieces of fruit. So your goal should be to find cleaner numbers to calculate.
You might try choice A, 3, which is very easy to calculate (23 oranges minus 3 = 20 oranges left), but a quick scan there would show that that’s way too many oranges (still more oranges than apples). So the other number that can clean up the 23 oranges is 17 (choice D), which would at least give you an even number (23 – 17 = 6). Because you’re now dealing with clean numbers (14, 6, and 70%) it’s worth doing the full calculation to see if choice D is really correct. And since 14 apples out of 20 total pieces of fruit is, indeed, 70%, you know that D is correct.
Now, if you follow these preceding paragraphs step-by-step, they should look just as long and unwieldy as the algebra or some traditional backsolving. But to an examinee seasoned in non-calculator math, finding “clean numbers worth testing” is more about the scan than the process. You should know that Odd + or – Odd = Even, but that Odd + or – Even is Odd. So with an even “fixed” number of 14 apples and an odd “changeable” number of 23 oranges, an astute GMAT test-taker looking to save time would probably eschew plugging in C first and realize that it’s just not going to be correct. Then another scan of numbers shows that only 3 and 17 are odd and prone to becoming “clean” when subtracted from the prime 23, so D should start looking tempting within seconds.
Note: this strategy isn’t for everyone or for every problem, but for those shooting for the 700s it can be extremely helpful to develop enough “number fluency” that you can save time not-testing numbers that you can see don’t have a real chance. On a non-calculator test that typically involves “clean” (even, divisible by 10, etc.) numbers, quickly recognizing which numbers will result in good, clean, non-calculator math is a very helpful skill.
By Brian Galvin