Question

There are 15 girls in your class this year. That is a 15% increase from last year. Write a proportion to solve for the number of girls in the class last year.

Answer 1

First, we have to recognize that if the current number of girls in the class is a 15% increase from last year, then this year's number is 115% of last year's number.

With that, we can set up a proportion comparing last year to this year, in both percentages and numbers:

`(100)/(115) = (x)/(15)`

Note that both numerators are last year's values and the denominators are this year's values.

If we multiply both sides by 15, we have that x ≈ 13.043478260869565. Since we're dealing with whole people, we'd say that there were 13 girls in the class last year.