A man swims downstream 35 km and upstream 20 km taking 5 hours each time, what is the speed of the man in still water? Choose the correct choice from the following:
A) 6.5	B) 5.5	C) 5.2	D) 4.6	E) 9.7

Rationale: "35 - - - 5 ds = 7 ? - - - - 1 20 - - - - 5 us = 4 ? - - - - 1 m = ? m = ( 7 + 4 ) / 2 = 5.5 answer : b"
Answer: B