Feb
6
2013

# Quick Multiplication and Division – How to Leave the Calculator Alone

GRE teachers have a love/hate relationship with the calculator that testers get on their quantitative section. It can be useful in certain situations – if you have to multiply 372 by 754, for example, then by all means take advantage of the calculator to get a quick answer. By and large, though, we find that the majority of our students, even the ones who use a lot of math as part of their jobs or classes, overuse the calculator on problems that they could solve much faster by hand. It’s not because they’re unintelligent or lazy – given how ubiquitous calculators and spreadsheets are in our daily lives now, a lot of people just don’t remember how to do quick scratch-paper arithmetic. (You’ll notice that I didn’t say “mental” arithmetic – do not do any steps in your head! Write your work down. But that’s a topic for another day.)

So, to help remedy this problem, here are some quick arithmetic tricks that you can use to save yourself time and energy on the GRE:

- To divide by 4: Divide the number by 2, and then divide by 2 again.

- To multiply by 4: Flip the last trick. Double the number, and then double again.

- To multiply by 5: Multiply the number by 10, and divide by 2. (For integers, this just means adding a 0 to the end of the number and taking half of the result.)

- To divide by 5: Again, flip the previous tip. Divide the number by 10 (take away a 0 or move the decimal point one unit to the left), and double it.

- Once you’ve multiplying and dividing by 4 and 5 down, working with other numbers becomes simpler as well: Multiplying by 6, for example, just involves doubling a number and then multiplying that by 3.

- To quickly find percentages, multiply the number by the integer value of the percent, and use logic to determine where the decimal point goes. For example: If you need to find 12% of 300, first multiply 12 and 300: That gives us 3600. But we’re looking for a value that’s just a bit bigger than 30 (which is 10% of 300), so our answer must be just 36.

Applying these tricks may feel like a poor use of time at first, but if you practice by doing just a couple of calculations a day this way, by Test Day you’ll be a scratch-paper math whiz – able to outpace both the calculator and everyone else in the testing room!

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#### About the Author: Teresa Rupp

Teresa Rupp has been a Kaplan GRE teacher since the beginning of 2010. She graduated from Georgetown University with a degree in Middle Eastern Studies, which has left her with an enduring love of Lebanese cuisine. When she’s not coaching students to Test Day success in Baltimore and in Kaplan’s Anywhere classes, Teresa can usually be found reading, doing crossword puzzles, or hiking with Piper, her Welsh Springer Spaniel (who also enjoys Lebanese food).

• Macinac

The key word here is “trick”. In my opinion it’s just as easy to learn direct arithmetic as remembering a set of “tricks”.

• Teresa Rupp

Hi Macinac! It entirely depends on someone’s particular strengths and how he or she learns best – the great thing about math is how many different ways there are to think through any one problem or set of calculations

• http://www.facebook.com/hookerg Geoffrey Hooker

To multiply by 6, double your number, record it, double that answer, and add the two together.

• Teresa Rupp

Thanks, Geoffrey