3
Find the missing part
Joyful Mathematics: Chapter 6
GoalFind how many more are needed to reach a number up to 20.
New words
missing · not there yethow many more · the number we still needhop · jump along the number line
Explanation
Sometimes a part is missing. 12 + ? = 15. Start at 12. Hop to 15. Count the hops: 3. So the missing number is 3. 12 + 3 = 15. We can count the hops to find the missing part.
Worked examples
9 + ? = 13. Start at 9. Hop to 13. You hop 4 steps. So 9 + 4 = 13.
7 + ? = 12. Hop from 7 to 12. You hop 5 steps.
14 + ? = 20. 14 and 6 more make 20.
Class activity
Top up to 20. Put 12 beans in a cup. Ask: how many more beans make 20? Add beans one at a time and say 13, 14, 15 up to 20. Count how many beans you added: 8. Try again from 15 and from 17.
Practice
1
The frames show 12 dots. How many more dots make 15?
3.
2
The frames show 16 dots. How many more make 20?
4.
3
Fill in: 8 + ___ = 14
6.
4
Imran has 11 marbles. He wants 18 marbles. How many more does he need?
7.
5
Fill in: 10 + ___ = 20
10.