A man swims downstream 54 km and upstream 18 km taking 3 hours each time, what is the speed of the man in still water? Options
A) 2
B) 8
C) 9
D) 10
E) 12

Rationale: "54 - - - 3 ds = 18 ? - - - - 1 18 - - - - 3 us = 6 ? - - - - 1 m = ? m = ( 18 + 6 ) / 2 = 12 answer : e"
Answer: E