请教GWD 数学

lbling417

In planning for a trip, Joan estimated both the distance of the trip, in miles, and her average speed, in miles per hour. She accurately divided her estimated distance by her estimated average speed to obtain an estimate for the time, in hours, that the trip would take.  Was her estimate within 0.5 hour of the actual time that the trip took?
(1) Joan's estimate for the distance was within 5 miles of the actual distance.
(2)   Joan's estimate for her average speed was within 10 miles per hour of her actual average speed.
答案是E
题目我看得有点晕晕的，选C为什么错啊？