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?

A. 15 kmph
B. 10 kmph
C. 12 kmph
D. 12.5 kmph
E. None of these
Explanation:
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.