Question

The period of a simple pendulum X is 5s l. What is the period of a simple pendulum Y which makes 50 vibrations in the same time it takes X to make 20 vibrations

Answer 1

The pendulum X makes 20 vibrations in $5s\times 20 = 100s$. If in this time the pendulum Y makes 50 vibrations, then its period is:

$$T_Y = \dfrac{100s}{50} = 2s$$

Answer. 2s.