Number Sentences and Missing Numbers
Find unknown values in numerical equations involving addition and subtraction, using the properties of numbers and operations. Use mathematical modelling to solve practical problems involving additive and multiplicative situations including financial contexts; formulate the problems using number sentences.
Read this first
A number sentence is a maths story. Find the missing part by working backwards.
8 plus something makes 14. The gap is 6.
Words you need
- Number sentence
- A maths sentence with an equals sign.
- Equals
- Both sides are worth the same.
- Unknown
- The part you do not know yet.
Worked example
e.g. Find the missing number: 8 + ? = 14
- Read it out loud. 8 plus something makes 14.
- Find the gap between 8 and 14.
- Count on from 8. 9, 10, 11, 12, 13, 14. That is 6 jumps.
- So the missing number is 6.
Always check. Put your answer back in. 8 + 6 = 14.
Practice
The pictures fade as you go. By the last few you are on your own.
1Find the missing number: 5 + ? = 12with a picture
Answer7.
2Find the missing number: ? - 4 = 10with a picture
Answer14.
3Find the missing number: 20 - ? = 13with a hint
Need a hint?
What is the gap from 13 to 20?
Answer7.
4Sam has 15 cards. He gets some more. Now he has 23. Write a number sentence.with a hint
Need a hint?
Use a question mark for the part you do not know.
Answer15 + ? = 23. He got 8.
5Find the missing number: ? + 9 = 21on your own
Answer12.
6A book costs 20. Write a number sentence for the change.on your own
Answer20 - 12 = ?. The change is $8.
7Check this: 7 + 8 = 14. Is it true?on your own
AnswerNo. 7 + 8 = 15.
Think about it
1
What does the equals sign really mean?
One good answerBoth sides are worth the same. It is not 'the answer comes next'.
2
Why write a number sentence for a word problem?
One good answerIt turns the story into maths you can solve.
Common mistakes
Thinking equals means 'write the answer'.
It means both sides balance.
Guessing the missing number.
Count the gap, or work backwards. Then check.