The speed of a boat along and against the current is $22\text{km/hr}$ and $18\text{km/hr}$ respectively. Then the speed of the current (in $\text{km/hr}$) is:

Given:
* Downstream speed ($B+C$) $=22\text{km/hr}$
* Upstream speed ($B-C$) $=18\text{km/hr}$

Where $B$ is the speed of the boat in still water and $C$ is the speed of the current.

The formula for the speed of the current is:

$$\text{Speed of Current }(C)=\frac{\text{Downstream Speed}-\text{Upstream Speed}}{2}$$

$$C=\frac{22-18}{2}=\frac{4}{2}=2\text{km/hr}$$