For this case what you should do is use the equation shown.
We have then:
n * A = 360
Where:
A=Angle
n: number of divisions.
We observe that there are eight divisions. Thus:
n = 8
We then clear the angle between each division
8 * A = 360
A = 360/8
A = 45 degrees
Answer:
The measure of the angle between the mirrors is:
A = 45 degrees