What Percent Is It

Sometimes you know the part and the whole and want the percent. You scored 12 out of 20 on a quiz: what percent is that? The part over the whole is the fraction $\frac{12}{20}$. A percent counts hundredths, so multiply the fraction by 100: $\frac{12}{20}\cdot 100$ is 60.

On the board, pick the letter p for the percentage, the number in front of the percent sign. Write $p = \frac{12}{20}\cdot 100$, then $p = 60$, which says 60%. Do not write the percent sign after it: on the board 60% is the number 0.6, a different value from 60.

Why does multiplying by 100 work? $\frac{12}{20}$ is 0.6, and 0.6 is 60 hundredths. Multiplying by 100 just counts those hundredths for you.