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 4 hours longer than the first half, how many hours did it take the runner to run the second half?  Choose one.
A) 6  B) 7  C) 8  D) 9  E) 10
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+4   T1 = 4 and so T2 = 8.   The answer is 8.
<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 4 hours longer than the first half, how many hours did it take the runner to run the second half?