A man swims downstream 30 km and upstream 18 km taking 6 hours each time, what is the speed of the man in still water? Choose the correct choice.
A) 7
B) 8
C) 5
D) 2
E) 4

Rationale: "30 - - - 6 ds = 5 ? - - - - 1 18 - - - - 6 us = 3 ? - - - - 1 m = ? m = ( 5 + 3 ) / 2 = 4 answer : e"
Answer: E