A family drove from home to a vacation destination 100 miles away, driving the first half of the distance at a constant

A family drove from home to a vacation destination 100 miles away, driving the first half of the distance at a constant speed of 50 miles per hour and the second half of the distance at a constant speed of 20 miles per hour. Returning home by the same route, they traveled at a constant speed of 30 miles per hour for the whole trip. What is the number of hours it took to drive from home to the vacation destination?

A. 2.5 hours
B. 3 hours
C. 3 1/3 hours
D. 3.5 hours
E. 4 hours

Answer: D

Source: Manhattan GMAT