The number of images formed by three mirrors AB, BC, AC forming an equilateral triangle as shown in figure. Then number of images formed is 4n. Find n value
  
 

 
let us consider mirrors AB and BC.
$Now\frac{{360}^0}{{60}^0}=6$
N = no.of images formed by the combination = 6 – 1 = 5
Combination of mirrors images
AB & BC  1,2,3,4,5
AC & BC  1’,2’,3’,4’,5’
AB & AC  1”,2”3”,4”5”
AB & AC  1”,2”,3”,4”,5”
These images and object will lie on a circle as show figure with an angular separation of ${72}^0.$Similarly, the other two combination of mirrors forms 5 images each. Also, one finds from symmetry that 5 and 5’, 1 and 5”, 1 and 1” coincide. No.of images formed by three mirrors AB, BC and AC = 5 x 3 – 3 = 12