The fireman must stop at some point of the riverbank on his way to the fire. If we reflect the fire in the riverbank then our problem is equivalent to finding the shortest path, via the riverbank, to the reflected fire. This is just a straight line, so we reflect this line back in the riverbank to get our actual path. This shortest path is characterised by the fact that itarrives and leaves the riverbank at the same angle. We say that the angle of incidence and the angle of reflection are the same.

Can you solve the second, seemingly unrelated problem, using the first?