Three identical plane mirrors AB, BC, AC are arranged as shown in the figure. Find the total number of images of a point object ‘S’ formed by the three mirrors. (S is at the centre of the system)

Number of images formed by every two adjacent mirrors $=\frac{360}{60}-1=5$. 

All the three mirrors form $3 \times 5 = 15$ images totally.

Among these, two images will coincide at three positions (P, Q, R). Hence total number of images appears to be $= 15 - 3 = 12.$
But the virtual images formed by one pair of mirrors will also become real objects to the other mirror. If we proceed in this way, number of images becomes infinity.