A man swims downstream 30 km and upstream 20 km taking 5 hours each time, what is the speed of the man in still water? Choose the correct answer
A) 7
B) 8
C) 2
D) 5
E) 4

Rationale: "30 - - - 5 ds = 6 ? - - - - 1 20 - - - - 5 us = 4 ? - - - - 1 m = ? m = ( 6 + 4 ) / 2 = 5 answer : d"
Answer: D