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 answers.
A) 6 B) 7 C) 8 D) 9 E) 10
91 --- 7 DS = 13   ? ---- 1   21 ---- 7 US = 3   ? ---- 1 M = ?   M = (13 + 3)/2= 
<gadget id="calculator">(13 + 3)/2</gadget>
<output>8</output>
8   Answer:8
<result>C</result>
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?