ôô

# How to Use Percentages to Easily Calculate Tips

Learn how to use the power of ten percent, and not your calculator, to easily and quickly calculate how much to tip.

By
Jason Marshall, PhD
Episode #12

## How to Easily Calculate Ten Percent

Because it leads us to the following quick and dirty tip: To find 10% of any number, all you have to do is divide that number by 10. Here’s what I mean. What’s 10% of \$10? Well, \$10 divided by 10 is \$1. So 10% of \$10 is \$1. How about 10% of \$20? Again, \$20 divided by 10 is \$2. So 10% of \$20 is \$2.

Here’s another quick and dirty tip to help you easily calculate 10% of any amount of money. First, write the amount out with the decimal point and the digits representing the number of cents—like you’d see on a cash register. For example: \$2.50 or \$85.00. Now, to find 10% of that amount, just move the decimal point one position to the left. (We’ll talk a lot more about decimal numbers in the future, so don’t worry if you’re not sure what the decimal point really means yet.) So, what’s 10% of \$2.50? Just move the decimal place one position to the left. The result is \$0.25, which is the same as 25 cents—or one quarter. What’s 10% of \$85.00? Again, move the decimal point one position to the left—the result is \$8.50.

## How to Quickly and Easily Calculate Tips

But why would any of this make you forget your tip-calculating app? Here’s the answer. At restaurants in the US, it’s generally accepted that good service warrants a tip between 15% and 20% of the bill. Let’s say your bill is \$34.00. How can you use the power of 10% to calculate the tip? First, calculate 10% of the bill: moving the decimal point one position to the left, 10% of \$34.00 is \$3.40. Now, if you want to tip 20%, just double this amount since 20% = 10% + 10% (or twice that of 10%). So the total tip would be 2 x \$3.40 = \$6.80.

If you wanted to leave 15% instead, you could use the fact that 15% = 10% + 5%. So since you already know that 10% of \$34.00 is \$3.40, you can just take half of that to figure out what 5% of \$34.00 is—\$3.40 / 2 = \$1.70. So a 15% tip would therefore be \$3.40 + \$1.70 = \$5.10. It might take a little practice at first, but soon you’ll find that the power of 10% allows you to quickly and easily calculate tips entirely in your head.

## Wrap Up

That’s all the math we have time for today. Please email your math questions and comments to mathdude@quickanddirtytips.com, follow the Math Dude on Twitter, and become a fan on Facebook. If you like what you’ve read and have a few minutes to spare, I’d greatly appreciate your review on the iTunes podcast store. And while you’re there, please subscribe to the podcast to ensure you’ll never miss a new Math Dude episode.

Until next time, this is Jason Marshall with The Math Dude’s Quick and Dirty Tips to Make Math Easier. Thanks for reading, math fans!

## Practice Problems

The only way to become proficient is to practice! Use the power of 10% to calculate the following tips:

1. 20% tip on a \$17 bill

2. 15% tip on a \$93 bill

3. 25% tip on a \$42 bill

4. 20% tip on a \$14.50 bill

5. 18% tip on a \$20 bill

Tip image from Shutterstock