A used book store will trade 2 of its books for 3 of yours. If Val brings in 18 books to trade, how many books can she get from the store?

You need to set it up as a fraction and make the fractions equal each other.

2/3 (2 of their books for 3 of yours)

x/18 (x amount of their books for 18 of yours)

(2/3) = (x/18)

multiply both sides by 18

and you get

36/3 which reduces to 12

so your answer is 12