Basic Algebra/Proportions and Proportional Reasoning/Percents

Vocabulary

Percent: Parts per one hundred

Lesson

Suppose we had a grid of 100 squares, of which 17 are shaded in. There are several ways to express this as a ratio, as you learned in the previous lesson, such as ${\frac {17}{100}}$, the number of shaded squares compared to the number of squares in total. However, we can also write this as a percent. Since there are 17 shaded squares and 100 total, we say that $17\%$ of the squares are shaded. The $\%$ symbol stands for "percent".

Now suppose that instead of 100 squares, we have 50 squares, with 9 of them shaded. The percent shaded would not be $9\%$, because percent means "per 100", and we have 9 shaded squares out of 50 total squares. To find the percent, we need some number over 100, so we can set up a proportion.

${\frac {9}{50}}={\frac {x}{100}}$

Cross-multiplying gives us $9\times 100=50x$, and $x=18$.

We also could have noticed that $50\times 2=100$, so we just need to multiply the numerator, 9, by 2 to get our answer, 18.

Practice Problems

Use / as the fraction line and put spaces between the wholes and fractions!

1 What percent of 91 is 137?

2 What is 260% of 70.5?

3 What percent of 109.5 is 49?

4 What percent of 135 is 81.7?