A man swims downstream 28 km and upstream 16 km taking 4 hours each time, what is the speed of the man in still water?	Choose the correct choice from the following answers:
A) 6.5 B) 8.6 C) 7.5 D) 9.2 E) 5.5

Rationale: "28 - - - 4 ds = 7 ? - - - - 1 16 - - - - 4 us = 4 ? - - - - 1 m = ? m = ( 7 + 4 ) / 2 = 5.5 answer : e"
Answer: E