Step by step
The question
Question: the distance-time graph joins (0, 0) (2, 80) (4, 80) (6, 0), find the average speed.
distance-time graph: (0, 0) (2, 80) (4, 80) (6, 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: 160 km.
Flat parts add nothing.
total distance = 80 + 80 = 160 km
The whole trip took 6 h, stops too.
total time = 6 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: 26.7 km/h.
average speed = 1606 ≈ 26.7 km/h
4 Check
Let's check: 26.7 times 6 gives back the total distance, 160.
So it is right!
26.7 × 6 ≈ 160
Let's look back.
First: read the graph.
Then: total distance and time.
Then: average speed.
Last: check.
Answer
average speed = 1606 ≈ 26.7 km/h