A man swims downstream 36 km and upstream 18 km taking 3 hours each time, what is the speed of the man in still water?
Choose the correct choice from the following options:
A) 7  B) 8  C) 9  D) 2  E) 4

Rationale: "36 - - - 3 ds = 12 ? - - - - 1 18 - - - - 3 us = 6 ? - - - - 1 m = ? m = ( 12 + 6 ) / 2 = 9 answer : c"
Answer: C