Step by step
The question
Question: the distance-time graph joins (0, 0) (2, 40) (3, 40) (4, 0), find the average speed.
distance-time graph: (0, 0) (2, 40) (3, 40) (4, 0)
1 Read the graph
Across: the time in hours.
Up: how far from the start, in km.
Going up: moving away from the start.
Flat: stopped.
Going down: coming back.
2 Total distance and time
Add up how far it went in each part, going and coming back: 80 km.
Flat parts add nothing.
total distance = 40 + 40 = 80 km
The whole trip took 4 h, stops too.
total time = 4 h
Careful!
The time when it stopped is part of the trip.
It goes in the total time.
Careful: the stops count in the time
3 Average speed
Average speed is the total distance divided by the total time: 20 km/h.
average speed = 804 = 20 km/h
4 Check
Let's check: 20 times 4 gives back the total distance, 80.
So it is right!
20 × 4 = 80
Let's look back.
First: read the graph.
Then: total distance and time.
Then: average speed.
Last: check.
Answer
average speed = 804 = 20 km/h