Sometimes a number is missing. Use what you know about true equations and thinking addition.
Key idea
Find the whole and the parts. If the whole is missing, add the parts. If a part is missing, take the known part away from the whole, or count up.
Why it works
Every addition or subtraction sentence is about a whole and two parts. Knowing which number is the whole tells you what to do.
Worked example: A missing part
What goes in the box? $8 + \square = 11$
- 11 is the whole. 8 is one part.
- Count up from 8 to 11: 3.
- $8 + 3 = 11$
Answer: 3
Worked example: Take away how many?
What goes in the box? $14 - \square = 9$
- 14 is the whole. 9 is the part left.
- From 9 up to 14 is 5.
- $14 - 5 = 9$
Answer: 5
Worked example: The start is missing
What goes in the box? $\square - 4 = 7$
- The box is the whole: what you started with.
- Put the 4 back: $7 + 4 = 11$.
- $11 - 4 = 7$
Answer: 11
Common mistake
Adding the two numbers you can see, and writing 19 in $8 + \square = 11$. Put your answer in to check: $8 + 19$ is not 11!
For grown-ups: After your child finds a missing number, ask them to read the whole sentence aloud with it filled in. Does it sound true?