At Trent's garage, 3 cases of brake fluid last 4 weeks. Trent has 5 cases. How much more brake fluid should Trent order so that he has enough for 13 weeks? Round your answer up to the nearest whole number.

3/4 = x/13 ... 3 cases to 4 weeks = x cases to 13 weeks

this is a proportion, so we cross multiply

(4) (x) = (13) (3)

4x = 39

x = 39/4 or 9 3/4 cases ... so he needs 9 3/4 cases for 13 weeks ...

and he already has 5 ...

so he needs

9 3/4 - 5 = 39/4 - 5/1 = 39/4 - 20/4 = 19/4 or 4 3/4 cases <==