Question 667729
In such problems you have to find the catchup distance and the catch up speed

Robber speed = 14 mph

he has a lead time of 10 minutes. = 1/6 hour.

when Marshall starts the robber is 1/6 * 14 = 14/6-> 7/3 miles away

Marshall speed = 15 mph

catchup speed = 15-14 = 1mph

time = d/r

t= (7/3) / 1

so Marshall will take 7/3 hours to catch him
= 2 hours and 20 minutes