1. **State the problem:** Convert 20 metres per second (m/s) to kilometres per hour (km/h) using the given number machine rule. 2. **Formula used:** To convert from m/s to km/h, multiply the speed in m/s by 3.6. 3. **Explanation:** This works because 1 metre per second equals 3.6 kilometres per hour. This is due to the fact that 1 km = 1000 m and 1 hour = 3600 seconds, so: $$1\ \text{m/s} = \frac{1000\ \text{m}}{1\ \text{km}} \times \frac{3600\ \text{s}}{1\ \text{hour}} = 3.6\ \text{km/h}$$ 4. **Calculation:** $$20\ \text{m/s} \times 3.6 = 72\ \text{km/h}$$ 5. **Answer:** 20 m/s is equal to 72 km/h.