Here number of images formed during each occassion is noted and added
Number of images formed = n = $(\frac{{360}^{\circ }}{\theta }-1)$

Total number of images = Number of images when angle between mirrors is at 30°, 60°, 90° and 120°
n = $(\frac{{360}^{\circ }}{{30}^{\circ }}-1)$ + $(\frac{{360}^{\circ }}{{60}^{\circ }}-1)$ + $(\frac{{360}^{\circ }}{{90}^{\circ }}-1)$ + $(\frac{{360}^{\circ }}{{120}^{\circ }}-1)$

n = 11 + 5 + 3 + 2 = 21