Reading the distances travelled from the graph, and the time taken, we simply put the numbers in the formula for speed:

Stage 1: speed = distance / time = 100 m / 10 s = 10 m/s

Stage 2: speed = distance / time = 50 m / 10 s = 5 m/s

Stage 3: speed = distance / time = 150 m / 20 s = 7·5 m/s