Determines the unknown number needed to make an equation true.

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- add
- To bring two or more numbers (or things) together to make a new total

+ - equation
- a statement declaring two mathematical expressions are equal
- number
- an arithmetical value, expressed by a word, symbol, or figure, representing a particular quantity and used in counting and making calculations and for showing order in a series or for identification. A quantity or amount.
- subtract
- to take away from a group or a number of things

- - unknown
- a number or value we do not know
- unknown number
- Also known as a variable