Answer
$12\ km$
Work Step by Step
1. Assume $d\ (km)$ is the distance that a person can see, and $h\ (m)$ is the height of the person above the surface, we have $d=k\sqrt {h}$, where $k$ is a constant.
2. Based on the given conditions, $18=k\sqrt {144}$, thus $k=\frac{18}{12}=\frac{3}{2}$
3. For $h=64\ m$, we have $d=\frac{3}{2}\sqrt {64}=12\ km$
