A boat takes six hours to travel a certain distance downstream and five hours to travel a certain distance upstream. The distance travelled…

Solution
Let speed of boat in still water = $x$ km/hr
Let distance travelled downstream = $2d$ km
=> Distance travelled upstream = $d$ km
Using, $time = \frac{distance}{speed}$
=> $6 = \frac{2d}{x + 4}$ ---------(i)
and $5 = \frac{d}{x - 4}$ ---------(ii)
Dividing eqn(i) from (ii), we get :
=> $\frac{6}{5} = \frac{\frac{2d}{x + 4}}{\frac{d}{x - 4}}$
=> $\frac{6}{5} = \frac{2 (x - 4)}{x + 4}$
=> $6x + 24 = 10x - 40$
=> $10x - 6x = 24 + 40 = 64$
=> $x = \frac{64}{4} = 16$ km/hr