Answer
The ray strikes the mirror a distance of $~~9.0~cm~~$ below the top of the mirror.
Work Step by Step
Note that the light ray reflects off the mirror at the same angle is it strikes the mirror.
We can use similar triangles to find the vertical distance $x$ above point B where the light ray strikes the mirror:
$\frac{x}{15~cm} = \frac{10 - x}{10~cm}$
$10x = 150-15x$
$25x = 150$
$x = 6.0~cm$
Since the mirror is $15~cm$ high, the ray strikes the mirror a distance of $~~9.0~cm~~$ below the top of the mirror.
