A man swims downstream 30 km and upstream 12 km taking 3 hours each time, what is the speed of the man in still water?
Options
A) 8  B) 7  C) 5  D) 2  E) 4

Rationale: "30 - - - 3 ds = 10 ? - - - - 1 12 - - - - 3 us = 4 ? - - - - 1 m = ? m = ( 10 + 4 ) / 2 = 7 answer : b"
Answer: B