If the speed in a downstream is $pkmph$, and speed in upstream is $qkmph$, then the speed of current is

Let speed at boat in still water $=xkm/hr$

Let speed of stream $=ykm/hr$

$\therefore$ given speed of boat in up stream $=q$

$\therefore$ $x-y=q\to \left(1\right)$

Given speed of boat in down stream $=(x+y)km/hr$

Solving $\left(1\right)\&\left(2\right)$

$\begin{array}{l}x+y=p\\ \underset{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}{{}_{-}x-{}_{+}y={}_{-}q}\\ 2y=p-q\end{array}$

$y=\frac{1}{2}(p-q)kmph$

$\therefore$ speed of current + (speed of stream)

$=\frac{1}{2}(p-q)kmph$