Car X traveled from city A to city B in 30 minutes. The first half of the ... ...

Car X traveled from city A to city B in 30 minutes. The first half of the distance was covered at 50 miles per hour, and the second half of the distance was covered at 60 miles per hour. What was the average speed of car X?
(A) 200 / 11
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Nova Math - Problem Set V: Word Problems