A boat can move upstream at 20 kmph and downstream at 35 kmph, then the speed of the current is?  Select the correct option:
A) 5
B) 6
C) 7.5
D) 4
E) 32
US = 20   DS = 35   M = (35 - 20)/2= 
<gadget id="calculator">(35 - 20)/2</gadget>
<output>15/2 = around 7.5</output>
7.5   Answer: 7.5
<result>C</result>
A boat can move upstream at 20 kmph and downstream at 35 kmph, then the speed of the current is?