Detour to Work

Mr. Parker's job is on a curvy side road next to a highway exit. He drives down the highway at 60 m.p.h., and at this rate he always gets to work at 7:30 a.m. However, at 7:15 because of a detour sign, he was forced to leave the highway and take the side road, driving at a steady speed of 40 m.p.h. for the rest of the way. He arrived at work15 minutes later than originally planned.

 

How many miles did he have to drive on the side road?