Calculations in physics work best in SI units: metres for distance, seconds for time and metres per second for speed. Many everyday values are not in SI units, such as kilometres per hour or centimetres per second, so you must convert them using ratios. A rate is an amount of change per unit of time, and speed is the rate at which distance changes.
One kilometre is 1000 m and one hour is 3600 s. To convert km/h to m/s, multiply by 1000 and divide by 3600. This is the same as dividing by 3.6. For example, 108 km/h is 108 ÷ 3.6 = 30 m/s. To go back, multiply by 3.6, so 10 m/s is 36 km/h.
Other conversions use the same idea. Divide centimetres by 100 to get metres, so 90 cm/s is 0.9 m/s. Multiply minutes by 60 to get seconds, so 5 minutes is 300 s. Convert every value before you substitute into an equation. For example, a cyclist who travels 3 km (3000 m) in 20 minutes (1200 s) has a speed of 3000 ÷ 1200 = 2.5 m/s.