A runner runs the 40 miles from Marathon to Athens at a constant speed. Halfway through the run she injures her foot, and continues to run at half her previous speed. If the second half takes her 5 hours longer than the first half, how many hours did it take the runner to run the second half? Choose the correct option:
A) 8
B) 9
C) 10
D) 11
E) 12

Rationale: the runner runs the first 20 miles at speed v and the second 20 miles at speed v / 2 . the time t 2 to run the second half must be twice the time t 1 to run the first half . t 2 = 2 * t 1 = t 1 + 5 t 1 = 5 and so t 2 = 10 . the answer is c .
Answer: C