A man swims downstream 36 km and upstream 48 km taking 6 hours each time, what is the speed of the man in still water? Select:
A) 5
B) 24
C) 7
D) 42
E) 6

Rationale: "36 - - - 6 ds = 6 ? - - - - 1 48 - - - - 6 us = 8 ? - - - - 1 m = ? m = ( 6 + 8 ) / 2 = 7 answer : c"
Answer: C