A man swims downstream 100 km and upstream 30 km taking 10 hours each time; what is the speed of the current? Choose the correct choice from the following answers:
A) 2.5  B) 3.5  C) 4.5  D) 5.3  E) 3.4
100 --- 10 DS = 10   ? ---- 1   30 ---- 10 US = 3   ? ---- 1 S = ?   S = (10 - 3)/2= 
<gadget id="calculator">(10 - 3)/2</gadget>
<output>7/2 = around 3.5</output>
3.5   Answer: 3.5
<result>B</result>
A man swims downstream 100 km and upstream 30 km taking 10 hours each time; what is the speed of the current?