A man swims downstream 30 km and upstream 18 km taking 6 hours each time, what is the speed of the man in still water?  Select the correct option:
A) 7 B) 8 C) 5 D) 2 E) 4
30 --- 6 DS = 5   ? ---- 1   18 ---- 6 US = 3   ? ---- 1 M = ?   M = (5 + 3)/2= 
<gadget id="calculator">(5 + 3)/2</gadget>
<output>4</output>
4   Answer:4
<result>E</result>
A man swims downstream 30 km and upstream 18 km taking 6 hours each time, what is the speed of the man in still water?