Rohit rows his boat 36 km downstream in 4 hours and 25 km upstream in 5 hours. Find the speed (in km/h) of his boat in still water.

Let the boat's still-water speed be $x$ km/h and the stream's speed $y$ km/h. Downstream: $x+y=36/4=9$. Upstream: $x-y=25/5=5$. Adding: $2x=14$, so $x=7$ and $y=2$. Check: 7 + 2 = 9 and 7 - 2 = 5. 2 is the stream's speed; 9 is the downstream speed $x+y$; 5 is the upstream speed $x-y$.