Probably my head is a bit slow today...
but this took me ages too:

Harry started a 6-mile-hike with a full 10-cup canteen of water and finished the hike in 2 hours with one cup of water remaining in the canteen. If the canteen leaked at the rate of 1 cup per hour and Harry drank 3 cups of water during the last mile, how many cups did he drink per mile during the first 5 miles of the hike?

A) 4/5
B) 5/6
C) 1
D) 6/5
E) 5/4