Step by step
The question
Question: the distance-time graph joins (0, 0) (1, 45) (2, 45) (4, 45) (5, 0), find the average speed.
distance-time graph: (0, 0) (1, 45) (2, 45) (4, 45) (5, 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: 90 km.
Flat parts add nothing.
total distance = 45 + 45 = 90 km
The whole trip took 5 h, stops too.
total time = 5 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: 18 km/h.
average speed = 905 = 18 km/h
4 Check
Let's check: 18 times 5 gives back the total distance, 90.
So it is right!
18 × 5 = 90
Let's look back.
First: read the graph.
Then: total distance and time.
Then: average speed.
Last: check.
Answer
average speed = 905 = 18 km/h