Question

how many images are formed for a point source kept in between two plane mirror m1 and m2 at right angles to each other

Answer 1

The formula for finding this is : - n=360/x or n=360/x - 1, where n is the no. of images, x is the angle of inclination between the two mirrors & n is always an odd integer.
So,if the angle between the two mirrors is 90 degrees, n=360/90 - 1= 4-1 =3

Answer 2

the formula is : n = 360/x-1
in the give formula the terms are :
1. n = number of the images formed
2. x = the angle 
 
given : mirrors are at 90 degree angles
therefore n = 360/x-1
n = 360/90-1
n = 4-1
n = 3
∴ the number of images formed are 3.