21 out of 24 students did their homework. What percent did NOT do their homework.

If 21 out of 24 students did their homework, 3 out of 24 did not do it.

This written as a fraction is 3/24 and can be reduced to 1/8.

This in decimal form is 0.125.

To convert the decimal to a percent, multiply by 100 and put a percent sign at the end of the number.

0.125 * 100 = 12.5

12.5%