A car travels from Mayville to Rome at an average speed of 30 miles per hour and returns immediately along the same route at an average speed of 40 miles per hour. Of the following, which is the closest to the average speed, in MPH, for the round-trip?

A. 32.0

B. 33.0

C. 34.3

D. 35.5

E. 36.5

(30 + 40)/2 = 35

So, 35 is the average speed for the round-trip.

From the choices given, the closest to 35 is 35.5.

The book's answer is C. Why is my answer wrong?