A man swims downstream 18 km and upstream 12 km taking 3 hours each time, what is the speed of the man in still water? Choose the correct choice from the following answers:
A) 3
B) 5
C) 6
D) 4
E) 8

Rationale: "18 - - - 3 ds = 6 ? - - - - 1 12 - - - - 3 us = 4 ? - - - - 1 m = ? m = ( 6 + 4 ) / 2 = 5 answer : b"
Answer: B