Consider the question. Police officer A patrols 36 streets in 4 hours. His colleague, officer B, patrols 55 streets in 5 hours. How many streets will both officers patrol in one hour?
 If the answer is '20'; explain the reasoning:
Officer A patrols 36 streets / 4 hours = 9 streets / hour. Officer B patrols 55 streets / 5 hours = 11 streets / hour. Together officer A and B patrol 9 streets / hour + 11 streets / hour = 20 streets / hour.