After robbing a bank in Dodge City a robber gallops off at 1

After robbing a bank in Dodge City, a robber gallops off at 12 mi/h. 20 minutes later, the marshall leaves to pursue the robber at 15 mi/h. How long (in hours) does it take the marshall to catch up to the robber?

Solution

r is 4mi ahead
overtake at 3 mph
4/3=1 1/3 hr or 1 hr 20 min