If it is from Ps Q, We can do like that

alfa=20d, beta=2d.

From second ,reflects 1 time, the angle increases 2d,so,n=36 the angle is 90d,that means light goes back the same road,the answer shoud be 72. 70,71,also OK,It is about what is first and last...

If it is strict Math, We have to think about the length,it is boring,...