On a trip, a cyclist averaged 9 miles per hour for the first 18 miles and 10 miles per hour for the remaining 12 miles. If the cyclist returned immediately via the same route and took a total of 7.2 hours for the round trip, what was the average speed (in miles per hour) for the return trip?	Select.
A) 6.9
B) 7.2
C) 7.5
D) 7.8
E) 8.1

Rationale: "the time to go 30 miles was 18 / 9 + 12 / 10 = 2 + 1.2 = 3.2 hours . the average speed for the return trip was 30 miles / 4 hours = 7.5 mph . the answer is c ."
Answer: C