A driver goes on a trip of 60 kilometers, the first 30 kilometers at 60 kilometers per hour and the remaining distance at 30 kilometers per hour. What is the average speed of the entire trip in kilometers per hour?
Choose the correct choice.
A) 35
B) 36
C) 40
D) 42
E) 45

Rationale: "the time for the first part of the trip was 30 / 60 = 1 / 2 hours . the time for the second part of the trip was 30 / 30 = 1 hour . the total time for the trip was 3 / 2 hours . the average speed for the trip was 60 / ( 3 / 2 ) = 40 kph the answer is c ."
Answer: C