WAEC Physics 2021 Paper

Concept:Each hole produces one pulse of sound when it passes the air jet.Explanation:Let the speed of rotation be $n$ revolutions per second.In one revolution, all $20$ holes pass the air jet.Therefore, the number of sound pulses produced per second is $20 \times n$.This gives the frequency of the note:$$20n = 480$$$$n = \frac{480}{20} = 24\text{ rev/s}$$Answer:The speed of rotation of the disc is $24\text{ rev/s}$. Hence, the correct option is C.