In a Certain One-Lane Road There Are No Entrances or Exits

Multiple Choice

In a certain one-lane road there are no entrances or exits. As the road passes through open country and populated areas, the speed limit changes. Assume that cars travel at the posted speed limit. In the open road, where the speed limit is 55 mph, the cars are spaced so there are 51 cars per mile. If the flow of cars is steady, how many cars per mile are there in a stretch of road where the speed limit is 40 mph?

A) 56 cars/mile
B) 60 cars/mile
C) 66 cars/mile
D) 70 cars/mile
E) 76 cars/mile