For a moving object, (average) speed = distance ÷ time. Use distance in metres, m, time in seconds, s, and speed in metres per second, m/s. A walker who covers 150 m in 50 s has an average speed of 150 ÷ 50 = 3 m/s.
The equation can be turned around. distance travelled = average speed × time. A car travelling at an average speed of 12 m/s for 20 s travels 12 × 20 = 240 m. To find the time, use time = distance ÷ speed. A cyclist who travels 600 m at 4 m/s takes 600 ÷ 4 = 150 s.
We say average speed because the speed of a real journey keeps changing. Make sure all the units match before you calculate. To change minutes into seconds, multiply by 60. To change hours into seconds, multiply by 3600.