A man can row three-quarters of a kilometer against the stream in 111/4 minutes and down the stream in 71/2 minutes. The speed (in km/hr) of the man in still water is:

Answer: Option D

We can write three - quarters of a kilometer as 750 meters and $$11\frac{1}{4}$$ minutes as 675 seconds
$$\begin{array}{rl}& \text{Rate}\text{upstream}\\ & =\frac{750}{675}m/\sec =\frac{10}{9}m/\sec \\ & \text{Rate}\text{downstream}\\ & =\frac{750}{450}m/\sec =\frac{5}{3}m/\sec \\ & \therefore \text{Rate}\text{in}\text{still}\text{water}\\ & =\frac{1}{2}\left(\frac{10}{9}+\frac{5}{3}\right)m/\sec \\ & =\frac{25}{18}m/\sec \\ & =\left(\frac{25}{18}\times \frac{18}{5}\right)km/hr\\ & =5km/hr\end{array}$$