A boat goes 100 km downstream in 10 hours, and 60 km upstream in 30 hours. The speed of the stream is?  Pick one:
A) 5km/h
B) 2km/h
C) 3km/h
D) (1/2) km/h
E) E)4
100 --- 10 DS = 10   ? ---- 1   60 ---- 30 US = 2   ? ----- 1 S = (10 - 2)/2= 
<gadget id="calculator">(10 - 2)/2</gadget>
<output>4</output>
4 kmph   ANSW4R:4
<result>E</result>
A boat goes 100 km downstream in 10 hours, and 60 km upstream in 30 hours. The speed of the stream is?