A simple pendulum completes 15 oscillations in 20 sec. Find its time period and frequency.

The time period is the time taken to complete one oscillation

Hence time period $\mathrm{T}=\frac{20}{15}=1.33\sec$

$\text{ and frequency }v=v=\frac{1}{T}=\frac{1}{1.33}=0.75 \mathrm{Hz}$