Two mirrors are at right angles to one another. A light ray is incident on the first at an angle of 30° with respect to the normal to the surface. What is the angle of reflection from the second surface?

reflected angle - secod mirror = 60°

Explanation:

I attached an image with the solution to this problem below.

In the solution the reflection law, incident angle = reflected angle, is used. Furthermore some trigonometric relation is used.

You can notice in the image that the angle of reflection in the second mirror is 60°

if you already have the answer to (a), then just work out how many periods there are in 1 minute, and that's the answer to (c).