3
Add and take away ten
Discover Mathematics P2: Term 1, Lessons 11–20
GoalAdd 10 or take away 10 using the number chart.
New words
number chart · rows of numbers, ten in each rowten more · one row down on the chartten less · one row up on the chart
Explanation
Look at the number chart. Each row has ten numbers. When you move one row down, you add 10. When you move one row up, you take away 10. The ones digit stays the same and the tens digit changes by one. So 4 + 10 = 14 and 16 − 10 = 6.
Worked examples
4 + 10: start at 4 and move down one row to 14. 4 + 10 = 14.
16 − 10: start at 16 and move up one row to 6. 16 − 10 = 6.
10 + 7: the order does not matter. Start at 7 and move down one row to 17.
Class activity
Ten hunt. Hide 10 spoons in two rooms. Say: there are 10 here; I add 10 more. How many now? Then take 10 away again and say how many are left. Count by tens: 10, 20, 30.
Practice
1
What is 5 + 10?
15
2
What is 10 + 9?
19
3
What is 18 − 10?
8
4
What is 20 − 10?
10
5
Hana has 12 stickers and gets 10 more. How many stickers does she have now?
22