Learning to do basic math in your head is useful, but there are times when you need to make sure you are calculating the problem exactly. To subtract any percentage from a number, simply multiply that number by the percentage minus 100. To divide by 20 percent, multiply by 80 percent. To divide by 30 percent, multiply the number by 70 percent.

Before beginning, determine the gross amount you want to use before subtracting a percentage. On many bills or estimates, you may prefer the gross total before taxes have been applied. Subtracting 20 percent off a number that has additional taxes and fees might skew your overall estimate.

## Using a Calculator

1. Enter the amount from which you want to subtract 20 percent on your calculator. As an example, assume the total is $85.50.

## Sciencing Video Vault

2 Press the multiplication button – x – on the calculator..

3. Enter “0.8” to represent 80 percent, then hit the equal button. In the example, $85.50 x 0.8 = $68.40.