A girl swims downstream 63 km and upstream 28 km taking 72 hours each time; what is the speed of the current?  Choose the correct choice from the following answers:
A) 0.5  B) 1.5  C) 2.1  D) 2.5  E) 5.2
63 --- 7 2.5S = 9   ? ---- 1   28 ----7 US = 4   ? ---- 1 S = ?   S = (9 - 4)/2= 
<gadget id="calculator">(9 - 4)/2</gadget>
<output>5/2 = around 2.5</output>
2.5   Answer: 2.5
<result>D</result>
A girl swims downstream 63 km and upstream 28 km taking 72 hours each time; what is the speed of the current?