In a real journey the speed is rarely steady. A car speeds up, slows down and stops at lights. This is non-uniform motion, because the speed changes during the journey. A speedometer shows the instantaneous speed, which is the speed at one moment.
To describe a whole journey with one number we use the average speed. Average speed is the total distance travelled divided by the total time taken: average speed = total distance ÷ total time. With distance in metres and time in seconds, the answer is in m/s.
The total time must include any time spent stationary. A cyclist rides 900 m in 150 s, then rests for 50 s. The total time is 150 s + 50 s = 200 s, so the average speed is 900 m ÷ 200 s = 4.5 m/s. Dividing by 150 s would give the speed while moving, not the average speed for the whole journey.