Step by step
The question
Question: the distance-time graph joins (0, 0) (1, 30) (2, 30) (3, 0), find the average speed.
distance-time graph: (0, 0) (1, 30) (2, 30) (3, 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: 60 km.
Flat parts add nothing.
total distance = 30 + 30 = 60 km
The whole trip took 3 h, stops too.
total time = 3 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 = 603 = 20 km/h
4 Check
Let's check: 20 times 3 gives back the total distance, 60.
So it is right!
20 × 3 = 60
Let's look back.
First: read the graph.
Then: total distance and time.
Then: average speed.
Last: check.
Answer
average speed = 603 = 20 km/h