A flat mirror lies on level ground between a boy and a tower. The boy, whose eyes are 1.6 m above the ground, stands 2 m from the mirror and sees the very top of the tower reflected in it. The mirror is 25 m from the foot of the tower. How tall is the tower?

A mirror reflects at equal angles, so the ray makes the same angle with the ground on both sides. That gives two similar right triangles: eye height over its ground distance equals tower height over its ground distance. So $1.6/2=h/25$, and $h=25\times 0.8=20$ m. Inverting the ratio gives $25\times 2/1.6=31.25$ m, which is the common error.