An woman swims downstream 81 km and upstream 36 km taking 9 hours each time; what is the speed of the current? Choose the correct choice
A) 0.5  B) 1.5  C) 2.5  D) 3.5  E) 4.5
81 --- 9 DS = 9   ? ---- 1   36 ---- 9 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>C</result>
An woman swims downstream 81 km and upstream 36 km taking 9 hours each time; what is the speed of the current?