A man swims downstream 60 km and upstream 100 km taking 10 hours each time, what is the speed of the man in still water?  Choose the most appropriate option.
A) 24	B) 42	C) 6	D) 8	E) 12

Rationale: "60 - - - 10 ds = 6 ? - - - - 1 100 - - - - 10 us = 10 ? - - - - 1 m = ? m = ( 10 + 6 ) / 2 = 8 answer : d"
Answer: D