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? Options:
A) 6.9
B) 7.2
C) 7.5
D) 7.8
E) 8.1
The time to go 30 miles was 18/9+12/10=2+1.2= 
<gadget id="calculator">2+1.2</gadget>
<output>3.2</output>
3.2 hours.   The average speed for the return trip was 30 miles/4 hours= 7.5 mph.   The answer is 7.5.
<result>C</result>
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?