A driver goes on a trip of 50 kilometers, the first 25 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 from the following answers.
A) 35  B) 36  C) 40  D) 42  E) 45

Rationale: "the time for the first part of the trip was 25 / 60 = 5 / 12 hours . the time for the second part of the trip was 25 / 30 = 5 / 6 hours . the total time for the trip was 5 / 12 + 5 / 6 = 15 / 12 = 5 / 4 hours . the average speed for the trip was 50 / ( 5 / 4 ) = 40 kph the answer is c ."
Answer: C