If the speed of boat in upstream is $10kmph$ and the speed of boat in down stream is  $12kmph$, then the speed of boat in still water is 

Let speed of boat in still water be $xkmph$

Let speed of stream be $ykmph$

Given speed of boat up stream $=10kmph$

Given speed of boat down stream $=12kmph$

$\therefore$ $\begin{array}{l}x-y=10\\ \underset{\_\_\_\_\_\_\_\_\_\_\_\_\_\_}{x+y=12}\\ 2x=22\end{array}$

$x=\frac{22}{2}=11$

$\therefore$ speed of boat in still water  $=11kmph$