Speeds are often given in kilometres per hour (km/h) or miles per hour (mph), which are non-SI units. In physics calculations we convert to the SI unit, metres per second (m/s).
There are 1000 metres in a kilometre and 3600 seconds in one hour. To change km/h into m/s you multiply by 1000 and divide by 3600, which is the same as dividing by 3.6. For example, 72 km/h is 20 m/s, and 18 km/h is 5 m/s.
For miles per hour, 1 mph is about 0.45 m/s. A speed of 30 mph is therefore about 13 m/s.
A rate such as speed is a ratio: speed = distance divided by time. A car that travels 150 m in 10 s has a speed of 15 m/s. Always convert distances to metres and times to seconds first.