For the first hour of work, Manolo can make face-masks at the rate of one every four minutes. Thereafter, he can make face-masks at the rate of one every six minutes. How many face-masks does Manola make in a four-hour shift?
He makes 60/4=15 in the first hour
He makes 60/6=10 in each subsequent hour
There are 4-1=3 more hours in his shift
He makes 15+10*3=45.