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 you want to remain. In other words, multiply by 100 percent minus the percentage you want to subtract, in decimal form. To subtract 20 percent, multiply by 80 percent (0.8). To subtract 30 percent, multiply the number by 70 percent (0.7).

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.

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

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