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 12 hours longer than the first half, how many hours did it take the runner to run the second half? Choose the correct choice.
A) 20 B) 22 C) 24 D) 26 E) 28

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 + 12 t 1 = 12 and so t 2 = 24 . the answer is c ."
Answer: C