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Even in an age of smartphones, crunching numbers quickly — and getting good at it — can give you snappy, useful answers to pressing real-life math questions such as, Is this shirt a good deal? Or How much should I tip my Uber driver? Here are some good mental math shortcuts to keep under your hat.

To sign your check in record time:

To calculate the amount of a 20 percent tip, calculate 10 percent (or remove the last digit) and double it. For instance, with a $42.50 bill, 10 percent is $4.25, and double it to get an $8.50 tip.

To ace sale shopping:

To find out how much you’ll pay for an item that’s a certain percentage off, first subtract the percent off from 100. So if it’s 30 percent off, use 70; 60 percent off, use 40, etc. Divide this number and the price by 10 and then multiply the resultant two numbers. For instance, if a $20 shirt is 60 percent off, multiply 2 by 4 for a total of $8. A $40 shirt at 30 percent off would be 4 multiplied by 7 for a total of $28 final price.

If you just want to calculate the amount that’s coming off (not the final price), divide both numbers by 10 and then multiply the resulting numbers. So 60 percent off $120 would be 6 x 12, for a total of $72 off.

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Photo by Bethany Robertson/Apartment Therapy

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