Question

How many no of images of a person is formed who is in a room whose two adjacent walls and ceiling are plane mirrors

Answer 1

Look at the problem by incrementally adding the mirrors.
When you add the first mirror, you get one image of the object. When you add the second mirror orthogonal to the first one, you will get 2 more images - one from the original object and one from the previous image.
Adding the third mirror similarly gives you 4 more images - one from the original object and 3 from the previous 3 images.
So there will be 7 images in total.

In general, if you add a mirror at some angle to the previous mirror, the new mirror will generate images from the original object and the previous set of images. These newly created images will get reflected by the first mirror to give more images, and the process continues till all the new images formed overlap with the existing images, and you don't get any new images. You can try it out taking angle between the two mirrors as 45 degrees or 60 degrees. You should get 7 and 5 images respectively. (If the angle between the mirrors is x degrees, where x divides 360, then the number of images formed are 360/x - 1.)