At a used book sale, 5 books cost $15. What is the cost per book? At this rate, how many books can you buy for $21?

Answer 1

^$15/_{5 books} = ^$3/_{1 book}

$3 * books = $21

Divide by $3 to both sides:

$3 * books / $3 = $21 / $3

books = 7

So you can buy 7 books with $21.