# Video: Identifying the Constant of Proportionality from a Graph

Emma walks to school every day. Find her speed from the distance-time graph.

01:27

### Video Transcript

Emma walks to school every day. Find her speed from the distance-time graph.

Notice on the 𝑥-axis, we have the time in minutes and on the 𝑦-axis, we have the distance in metres. We are asked to find her speed. And speed is equal to distance divided by time. So we can go through and pick any distance, say 400. And then we divide by the corresponding time.

So we go over to our line. And we find that the 400 meters goes with the five minutes. And now, we simplify. 400 divided by five is 80. So this means Emma’s speed would be 80 meters per minute. Now, we also could have used a few other points. However, there aren’t very many other points that go through the light grey lines on our graph.

For example, if we would pick 200 meters and then try to find what time went with that, we would see that it would be around two and a half. But we couldn’t exactly guess. It’s better to have an exact amount to land on. So this is why it’s good to choose the 400 meters and the five minutes.

Overall, once again, Emma’s speed was 80 metres per minute.