A girl swims downstream 40 km and upstream 16 km taking 4 hours each time, what is the speed of the girl in still water? Choose the correct choice from the following answers.
A) 3	B) 4	C) 5	D) 6	E) 7
40 --- 4 DS = 10   ? ---- 1   16 ---- 4 US = 4   ? ---- 1 M = ?   M = (10 + 4)/2= 
<gadget id="calculator">(10 + 4)/2</gadget>
<output>7</output>
7   Answer:7
<result>E</result>
A girl swims downstream 40 km and upstream 16 km taking 4 hours each time, what is the speed of the girl in still water?