1
Review: graphs and mental math
Discover Mathematics P2: Term 1, Lessons 2–60
GoalReview reading bar graphs and pictographs, and mental strategies such as doubles, counting on and adding or taking away ten.
New words
bar graph · a graph that uses bars to show how manypictograph · a graph that uses small pictures; the key tells what each picture meansdouble · add a number to itselfcount on · start at the bigger number and count up
Explanation
In a bar graph, find the top of a bar and read the number beside it. In a pictograph, count the pictures and use the key: if each picture means 2, then 3 pictures mean 6. For mental math, remember doubles, such as 7 + 7 = 14, and near doubles, such as 7 + 8 = 15 (one more than 14). To add a small number, start at the bigger number and count on: 48 + 3 means 49, 50, 51. To add 10, move one row down on the number chart. To take away 10, move one row up.
Worked examples
In the graph above, the banana bar reaches 9, so 9 children chose bananas. Mango and guava together: 6 + 7 = 13 children.
Each picture means 2 books. Monday has 3 pictures: 2, 4, 6 books. Tuesday has 5 pictures: 10 books. Wednesday has 4 pictures: 8 books.
48 + 3: start at 48 and count on 49, 50, 51. The sum is 51.
Class activity
Family vote. Ask 10 people which they like best: tea, juice, milk or karkade. Make tally marks, then draw a bar graph on squared paper. Write two questions about it and ask someone to answer them: ‘Which got the most?’ and ‘How many more chose tea than milk?’
Practice
1
How many more children chose dogs than cats?
8 − 5 = 3 more.
2
Each picture means 2 flowers. How many flowers in all?
2 + 5 + 3 = 10 pictures, so 20 flowers.
3
What is 7 + 7? What is 7 + 8?
14 and 15.
4
What is 10 more than the shaded number? What is 10 less?
10 more is 73. 10 less is 53.
5
Habiba has 8 stickers and wants 15. How many more does she need? 8 + __ = 15.
7 more, because 8 + 7 = 15.