Formula for number of images = 360n−1\; \frac{360}{n} - 1n360−1
where n is the angle of inclination.
Given the number of images = 11, we have
360n−1=11\; \frac{360}{n} - 1 = 11n360−1=11
360n=12\; \frac{360}{n} = 12n360=12
n= 36012=30∘n = \; \frac{360}{12} = 30^{\circ}n=12360=30∘
