1. **State the problem:** A pendulum makes 40 vibrations in 20 seconds. We need to calculate its period. 2. **Formula:** The period $T$ of a pendulum is the time taken for one complete vibration. It is given by: $$T = \frac{\text{total time}}{\text{number of vibrations}}$$ 3. **Apply the formula:** $$T = \frac{20}{40}$$ 4. **Simplify the fraction:** $$T = \frac{\cancel{20}}{\cancel{40}} = \frac{1}{2}$$ 5. **Interpretation:** The period of the pendulum is $\frac{1}{2}$ seconds, meaning it takes half a second for one vibration.