A man swims downstream 40 km and upstream 30 km taking 5 hours each time, what is the speed of the man in still water?
Choose the correct choice from the following options
A) 12	B) 8	C) 36	D) 5	E) 7

Rationale: "40 - - - 5 ds = 8 ? - - - - 1 30 - - - - 5 us = 6 ? - - - - 1 m = ? m = ( 8 + 6 ) / 2 = 7 answer : e"
Answer: E