Subtraction can represent finding the unknown part of a whole when the whole and one part are known: in , the missing part is the number that makes . Part–part–whole diagrams, equations, and number lines show that addition and subtraction describe the same relationship; this understanding applies to whole numbers within 20, not to larger-number algorithms or generalized algebraic treatment.
When you know the whole and one part, subtraction can help you find the missing part.
Example:
The whole is .
One part is .
Ask: “What number added to makes ?”
So, the missing part is :
This means that and are the two parts that make the whole :
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