Percent of a Number

What is 20% of 45? In mathematics, of means multiply, so 20% of 45 is $20\%\cdot 45$. On the board, a percent is a number like any other, so you can write it exactly like that.

To work it out, write the percent as a fraction over 100: $\frac{20}{100}\cdot 45$. Now multiply the top by 45 and divide by 100: 20 times 45 is 900, and 900 divided by 100 is 9. So 20% of 45 is 9.

The answer makes sense: 20% is a fifth of the whole, and a fifth of 45 is 9. Checking a percent against a simple fraction like this, a half, a quarter or a tenth, catches most slips before you press Check.