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 answer:
A) 8 B) 9 C) 10 D) 11 E) 12
The runner runs the first 20 miles at speed v and the second 20 miles at speed v/2.   The time T2 to run the second half must be twice the time T1 to run the first half.   T2 = 2*T1 = T1+5   T1 = 5 and so T2 = 10.   The answer is 10.
<result>C</result>
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?