You have 90 minutes to complete atleast 18 chemistry and math problems. It takes 5 minutes to complet a chemistry problem and 4 minutes for each math problem. Which is a possible solution to this equation?

5C + 4M = 90.

C = chemistry

M=math

The task is doable because, at worst, all of the problems are in Chemistry, and that would require 5 times 18 = 90 minutes, leaving no extra time. But if there are n math problems, you'll save n minutes (finish n min. early).