A man swims downstream 42 km and upstream 18 km taking 3 hours each time, what is the speed of the man in still water?  Select:
A) 2 B) 8 C) 9 D) 6 E) 10

Rationale: "42 - - - 3 ds = 14 ? - - - - 1 18 - - - - 3 us = 6 ? - - - - 1 m = ? m = ( 14 + 6 ) / 2 = 10 answer : e"
Answer: E