Mexine can stack 20 boxes in 15 minutes. how long will it take her to stack 100 boxes

Lets use a rule of three simple with direct proportion to solve:
 20 boxes > 15 min
100 boxes > x
x = (100)(15)/20
x = 75
therefore it takes 75 mins to stack 100 boxes

If we use fractions to solve:
15/20 = x/100
and solve for x.
20/15 = 100/y
solve for y