A man swims downstream 51 km and upstream 18 km taking 3 hours each time, what is the speed of the man in still water?  Choose the correct option
A) 2
B) 8.5
C) 9.5
D) 6.5
E) 11.5

Rationale: "51 - - - 3 ds = 17 ? - - - - 1 18 - - - - 3 us = 6 ? - - - - 1 m = ? m = ( 17 + 6 ) / 2 = 11.5 answer : e"
Answer: E