A man rows a boat a certain distance downstream in 9 hours, while it takes 18 hours to row the same distance upstream. How many hours will it take him to row $$\frac{3}{5}$$ of the same distance in still water?

$$\eqalign{ & {\text{Speed of boat}} = x \cr & {\text{Speed of stream}} = y \cr & {\text{Distance}} = 9\left( {x + y} \right) = 18\left( {x - y} \right) \cr & x + y = 2x - 2y \cr & x = 3y \cr & {\text{Distance}} = 9\left( {x + y} \right) \cr & = 9\left( {3y + y} \right) \cr & = 36y \cr & 36y \times \frac{3}{5} = t \times x \cr & 36y \times \frac{3}{5} = t \times 3y \cr & t = \frac{{36}}{5} = 7.2 \cr} $$