The current in a river is 6 mph. A boat can travel 20 mph in still water. How far up the river can the boat travel if the round trip is to take 10 hours? Choose the correct choice from the following
A) 69 miles  B) 88 miles  C) 96 miles  D) 91 miles  E) 112 miles

Rationale: "upstream speed = 20 - 6 = 14 mph downstream speed = 20 + 6 = 26 mph d / 14 + d / 26 = 10 hours solving for d we get d = 91 answer : d"
Answer: D