Discover Mathematics P5: Term 2, Unit 10, Lessons 9–11
GoalRead a graph to find values, compare points, find changes and make predictions.
New words
interpret · to explain what a graph tells usincrease · a change that makes the value biggerpredict · to say what will probably happen next
Explanation
To read a graph, look at the labels and the scale first. To find a value, go from the point across to the vertical axis, or down to the horizontal axis, and read the number, using the scale. To find a change, subtract the two values: a rise from 20 to 35 is an increase of 35 − 20 = 15. When two points are at the same height, the value did not change between them. A steady rate gives a straight line, so you can predict by extending the line: if a tank gains 10 litres every minute, then after 5 minutes it has 50 litres. Always answer with the unit, such as litres, degrees or LE, and check that your answer fits the graph.
Worked examples
The graph shows water filling a tank. After 3 minutes the tank holds 30 litres. It gains 10 litres every minute, so the tank will hold 50 litres after 5 minutes.
The graph shows the temperature during six hours of the day. The hottest point is at hour 5: 35 °C. From hour 2 to hour 3 the temperature stayed at 25 °C. From hour 5 to hour 6 it went down by 35 − 30 = 5 °C.
The graph shows the price of guavas: 20 LE for 1 kg, 40 LE for 2 kg and so on. The points are on a straight line, so each kilogram costs 20 LE. Five kilograms would cost 5 × 20 = 100 LE.
Class activity
Graph detectives. Each child draws a simple graph with a scale, for example the temperature at four times of a day or the money saved in four weeks. Then the child hides the numbers in the table and swaps with a friend, who must read at least three values from the graph, find one change, and make one prediction.
Practice
1
Use the tank graph: how many litres are in the tank after 2 minutes?
20 litres.
2
Use the temperature graph: between which hours did the temperature stay the same?
Between hours 2 and 3 (25 °C).
3
Use the guava graph: what is the price of 3 kg? What would 6 kg cost?
3 kg cost 60 LE. 6 kg would cost 6 × 20 = 120 LE.
4
Use the temperature graph: how many degrees warmer is the hottest hour than the first hour?
The hottest is 35 °C and the first hour is 20 °C: 35 − 20 = 15 °C warmer.
5
Hana cycles 20 km in the first hour, 40 km after two hours and 60 km after three hours. How far will she cycle in 4 hours if she keeps the same speed?
She cycles 20 km every hour, so in 4 hours she cycles 4 × 20 = 80 km.
