A man can row 30 km downstream and 20 km upstream in 4 hours. He can row 45 km downstream and 40 km upstream in 7 hours. Find the speed of man in still water?
Choose the correct answer.
A) 12.8 kmph  B) 23.9 kmph  C) 21.9 kmph  D) 12.5 kmph  E) 13.8 kmph
Let the speed of the man in still water be a kmph and let the speed of the stream be b kmph.   Now 30/(a + b) + 20/(a - b) = 4 and 45/(a + b) + 40/(a - b)   = 7   Solving the equation, the speed of man in still water is   12.5 kmph.   Answer: 12.5 kmph
<result>D</result>
A man can row 30 km downstream and 20 km upstream in 4 hours. He can row 45 km downstream and 40 km upstream in 7 hours. Find the speed of man in still water?