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? Give me reasons, before answering the question
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. So the final answer is 45.