A boy swims downstream 91 km and upstream 21 km taking 7 hours each time, what is the speed of the boy in still water? Choose the correct choice from the following choices:	A) 6	B) 7	C) 8	D) 9	E) 10

Rationale: 91 - - - 7 ds = 13 ? - - - - 1 21 - - - - 7 us = 3 ? - - - - 1 m = ? m = ( 13 + 3 ) / 2 = 8 answer : c
Answer: C