Photo by

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.

## To see your future without a crystal ball:

Use the rule of 72 to calculate how long it would take an investment to double. The rule of 72 is that an investment that earns 10 percent interest will double in 7.2 years. Use this as a starting point for calculating various interest rates and lengths of time, by dividing the number 72 by your interest rate. For instance, if you are investing at a more conservative rate of 5 percent, you’d divide 72 by 5 for a total of 15 years (rounded up) for your money to double.

To figure out how long it would take your money to triple, use 115 instead of 72. So at an interest rate of 3 percent, it would take 38 years (115/3), for your initial amount to triple.

## To do fast house math:

To calculate how much a month more you’ll pay in a mortgage payment for a certain increase in house price, figure roughly 6 dollars per month for every thousand dollars more in total price. So for a house price that’s $30,000 more than another, you’ll pay $140 more dollars a month (for a 30-year mortgage at 5 percent interest).

## To translate temperatures when you travel:

For quick conversions between Fahrenheit and Celsius: Celcius x 2 + 30 = Fahrenheit and (Fahrenheit – 30) / 2 = Celsius.

Photo by

## To be the fastest calculator in any room:

Multiply any number by four quickly by doubling the number twice. So 106 times four is 212 plus 212, for a total of 424.

To multiply by five, multiply by ten and then divide in half. So 250 multiplied by five could by calculated as 2,500 divided by 2 for a total of 1,250.

## To ease your wage wonders:

Convert an hourly wage to a salary by using the formula that $1 per hour is $2,000 per year. So if you make $10 per hour, that would translate to a $20,000 per year salary.

Another way to compare hourly wages to a salary is to drop the three zeros of a salary and then divide by two. So a salary of $42,000 would be roughly equivalent to $21 an hour. Conversely, multiply an hourly rate by two and add three zeros to the total to figure out a salary from an hourly wage.

## To analyze the impact of that coffee habit:

To estimate the annual cost of a daily habit, multiply to find the weekly total amount you spend (so, times 5 if it’s a weekday habit, or times 7 if it’s every day a week. Then add two zeros to that number, and divide by two. So a $12 a day lunch five days a week will roughly set you back a whopping $3,000. (Just don’t do it.)

## To know how hard you have to study:

To quickly and roughly figure out what number you need to make a certain average, add together how far off you were from your target in each instance. Then add that resulting number to your target average for your final chance. For instance, if your scores are 90, 90, 85, and 95 and you’re hoping for a total average of 89, you’d add 1+1-4+6 for a total of 4. So you’d need to make 89-4, or 85, on your final test.

*Shifrah Combiths has been writing professionally for twenty years. She loves lifestyle photography, memory keeping, gardening, reading, and going to the beach with her husband and children.*