When 2 plane mirrors are set at right angles to each other and a coin is placed in-between these two plane mirrors, then three images will be formed, see the diagram below: 

The formula for calculating the number of images, when two are kept at an angle θ, is given as: (360/θ)−1

Now,θ = 90 degree.

Thus, the number of images formed will be

= (360/θ)−1

= (360/90)-1

= 4-1  

= 3