A boat can move upstream at 20 kmph and downstream at 35 kmph, then the speed of the current is?  Choose the correct choice from the following answers:
A) 5 kmph
B) 7.5 kmph
C) 8 kmph
D) 9 kmph
E) 4 kmph
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 kmph
<result>B</result>
A boat can move upstream at 20 kmph and downstream at 35 kmph, then the speed of the current is?