Speed is often given in kilometres per hour (km/h) or in centimetres per second, but the SI unit is metres per second (m/s). Before using an equation we convert to SI units.
Useful conversions: 1 km = 1000 m, 1 m = 100 cm, 1 minute = 60 s and 1 hour = 3600 s. So 3 km = 3000 m, 2.5 km = 2500 m, and 2 hours = 7200 s.
To convert km/h to m/s, multiply by 1000 to change kilometres to metres, then divide by 3600 to change hours to seconds. This is the same as dividing the km/h value by 3.6. For example, 54 km/h = 54 × 1000 ÷ 3600 = 15 m/s. To go the other way, multiply by 3.6.
A speed is a rate: the amount of distance covered per unit of time. Comparing quantities in the same units, using ratios and proportion, is the only fair way to compare them.