Timing is a big issue for many on the GRE math section. One way in which students could end up saving precious minutes is by using mental math shortcuts, the most powerful of which is known as approximation. What I’ve noticed, however, is some students feel they have to set up some sort of proportion to solve a problem, a step that is both tedious and fraught with the potential for arithmetic errors. So, instead of trying to find the exact answer to a problem by furiously crunching numbers on the paper, learn to approximate and avoid running out of time (as well as a case of hand cramps).
When using approximation to solve a problem, either round numbers up or down so that you get a nice, round number to work with. Rounding numbers to the nearest five or zero usually works well. The key to approximating is to make sure the numbers you round to don’t deviate too much from the original numbers.
For instance, let’s take a look at the problem below:
1. What is 49% of 23?
Many students feel the need to set up an equation and solve for an exact value. For the GRE, you do NOT want to do this. Yes, such an injunction may seem counterintuitive, especially for those who vividly recall their high school math days. Whereas getting an exact value usually led to an A in this context, on the GRE an exact value—and the attendant setting up of an equation or proportion—will only slow you down. That is why you want to approximate, or estimate, wherever possible. The gist is to avoid solving the problem. If the answer choices are really close, then, and only then, set up an equation/proportion to get a more exact number.
So let’s take a look at the question above, and let me re-word it. What is 50% of 23? Easy, right? The answer is half of 23, which is 11.5. Notice I rounded up a little bit. But the answer is going to be very close to 11.5. Again, the essence of approximating is to pick a number that is close to any of the numbers and one that is easy to work with. Rounding 23 to 25 still leaves you with a 49. If you also round the 49 to 50, then you start to deviate a little too much from the original problem. With the spread of the answer choices above, you can still get the correct answer by multiplying .50 x 25 and then choosing the closet answer less than your result.
In the end, either approach will yield (C) 11.27. As long as you are able to do mental math quickly, you can make a 1-minute problem a 15-second one. Savvy students may even solve the problem in less time, if they see that when you multiply .49 x 23, the number has to end in a seven. The only answer choice that has a 7 at the end is (C).
Now, if you’re not used to solving in your head, or approximating/estimating in general, my advice is to start with easy problems. Just as importantly, drop that pen or pencil. That’s right—think of the pen(cil) as a crutch that’s only preventing you from walking, so to speak.
Okay, let’s try another type of problem and one you are very likely to see on the data interpretation (this is the section with those pesky pie/bar/curve graphs). Dispensing with the actual graph and winnowing the problem down to its basics, here is an example. And remember—don’t use your pen!
14% of a company’s inventory shipped. If the company held on to 170,280 items then approximately how many items did they ship?
This problem is much trickier than the first one. To start, we want to note the difference between the items shipped (14%) and the items not shipped (86%). Next, we want to ask ourselves, what’s a nice round number that’s not too far from 86%? What about 80%? Well, look at the answer choices. Notice that they are actually pretty close together. Using 80% for 86% would significantly change the outcome. So let’s try another number closer to 86%. How about 85%.
Now, let’s return to the information in the problem. 170,280 items can be changed to 170,000 items. We can use the 85% to represent the 170,000 items that did not ship. What is the relation between 170,000 and 85%? Notice that 170 is double 85. So, if we double the percent and multiply by 1,000 we get the total number of items that didn’t ship. So, the total number of items that shipped is double 14%, which is 28, multiplied by 1,000, which equals 28,000 (remember the number of items is in thousands, i.e. 170 thousands or 170,000).
Note that the question asks to approximate. 28,000 is not the exact answer but is very close, and hence an approximation. Had we forgotten that we’d approximated in the beginning by changing 86 percent to 85 percent, and thus 14 percent to 15 percent, we could have mistakenly chosen answer 30,000, choice (D).
The takeaway is to solve problems by approximating. Doing so will save you lots of time, while not taxing your brain scribbling tedious math at the margins of your scratch paper. Remember, approximating well takes practice, but once mastered will pay huge dividends in terms of the time saved on the actual exam.