Question 21332
Another way to look at this problem is by using relative speed. If the robbers are going 12 mph while the marshal is going 16mph, then the marshal has a relative speed of 16-12=4mph. 

I convert 20 minutes to 1/3 an hour by putting 20/60=2/6=1/3.

If the robbers is going 12mph, and left 20 minutes before the marshal, then they have traveled (12/1)*(1/3)=4 miles before the marshal left. 

Marshal has a relative speed of 4 mph while the robbers have a 4 mile head-start. 4/4=1. Answer: 1 hour.