A man swims downstream 30 km and upstream 12 km taking 3 hours each time, what is the speed of the man in still water?  Select.
A) 8
B) 7
C) 5
D) 2
E) 4
30 --- 3 DS = 10   ? ---- 1   12 ---- 3 US = 4   ? ---- 1 M = ?   M = (10 + 4)/2= 
<gadget id="calculator">(10 + 4)/2</gadget>
<output>7</output>
7   Answer:7
<result>B</result>
A man swims downstream 30 km and upstream 12 km taking 3 hours each time, what is the speed of the man in still water?