Lesson 4: Percent

Property

A percent is a ratio whose denominator is 100.

Examples

• 57 out of 100 students is 57%, which is the ratio $\frac{57}{100}$.

• 25% means a ratio of $\frac{25}{100}$.

Property

To calculate a percent of a number:

1. Change the percent to a decimal fraction.
2. Multiply the number by the decimal fraction.

To change a percent to a decimal fraction, divide the percent by 100, or move the decimal point two places to the left.

Examples

• To find 8% of 300, convert 8% to a decimal by moving the decimal point two places left: $8\% = 0.08$. Then multiply: $0.08 \times 300 = 24$.
• A phone costs 850 dollars and sales tax is 7.5%. Convert 7.5% to a decimal: $0.075$. The tax is $0.075 \times 850 = 63.75$ dollars.
• Calculate 150% of 60. First, change 150% to a decimal: $1.50$. Then, multiply: $1.50 \times 60 = 90$.

Explanation

To use a percent in a calculation, you must first convert it into a form your calculator understands, like a decimal. Moving the decimal point two places left is a quick shortcut for dividing by 100.