A man of height $r$ stands at distance $p$ metres from a light-post and casts a shadow of length $s$. The height of the light-post is—

By similar triangles, the man and his shadow give $\dfrac{r}{s}=\dfrac{H}{p+s}$, where $p+s$ is the distance from the post to the shadow tip. Solving for the post height gives $H=\dfrac{r(p+s)}{s}$.