Chandra checks 1 + tan² a = 1/(cos² a) on the angles 0°, 15°, 30°, …, 345°…

Step 1. Count. $\dfrac{360}{15} = 24$ angles, from $0^\circ$ to $345^\circ$.

Step 2. Skip. $\cos a = 0$ only at $90^\circ$ and $270^\circ$, both in the list. That is $2$ angles.

Step 3. The value. $\tan 30^\circ = 0.5774$, so $1 + 0.3333 = 1.3333$. Also $\dfrac{1}{\cos^2 30^\circ} = \dfrac{1}{0.75} = 1.3333$.