Percent of a Total

A class records what each student chose for lunch:

What percent chose pizza? First find the total: the Students column adds to $10+6+9$, which is 25. Then take the part over the total and multiply by 100: $\frac{10}{25}\cdot 100$ is 40, so 40% chose pizza. It is the same step as turning a chance into a percent, because the part over the total is a fraction of the whole.

Why a percent? It compares groups of different sizes on one scale. 10 of 25 and 12 of 40 are hard to compare, but 40% and 30% are not. And the percents of all the rows add up to 100%, since together the rows make the whole: here 40%, 24% and 36%. That makes a handy check.