Acar leaves town traveling 40 mi/h. two hours later, a second car leaves the same town, on the same road, traveling at 60 mi/h. in how many hours will the second car pass the first car? use formula d=rt

Don't really know how to use that formula, but the second car will past the first after 4 hours. 
1      240 = 40(6) 
2      240 =60(4)

First car has a "t" of 6, because the second car traveled two hours later.

Ithink the answer is c