A boatman can row a boat downstream and upstream at $13\text{kmph}$ and $9\text{kmph}$ respectively. What will be the speed of boat in still water? (in $\text{kmph}$)

Let the speed of the boat in still water be $x\text{kmph}$ and the speed of the stream be $y\text{kmph}$.
We are given:

* Downstream speed ($x+y$) = $13\text{kmph}$
* Upstream speed ($x-y$) = $9\text{kmph}$

The formula for the speed of the boat in still water is:

$$\text{Speed in still water }(x)=\frac{\text{Downstream Speed}+\text{Upstream Speed}}{2}$$

$$x=\frac{13+9}{2}=\frac{22}{2}=11\text{kmph}$$