A man can row threequarters of a kilometre against the stream in 45/4 min and return in 15/2 min. The speed of the man in still water is :
$$speed = \frac{distance}{time}$$
=> Upstream speed = $$\frac{\frac{3}{4}}{\frac{45}{4} \times \frac{1}{60}}$$
= $$\frac{3 \times 60}{45} = 4$$ km/h
Downstream speed = $$\frac{\frac{3}{4}}{\frac{15}{2} \times \frac{1}{60}}$$
= $$\frac{3 \times 2 \times 60}{15 \times 4} = 6$$ km/h
$$\therefore$$ Speed of man in still water = $$\frac{1}{2}$$ (downstream + upstream)
= $$\frac{1}{2} (6 + 4)$$
= $$\frac{10}{2} = 5$$ km/h
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