Answer:
x + 7
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What are variables?
A variable is a quantity that may change within the context of a mathematical problem or experiment. Typically, we use a single letter to represent a variable. The letters x, y, and z are common symbols used for variables!
How do we describe variables in word form?
When we describe an expression in words that includes a variable, we're describing an algebraic expression (an expression with a variable).
In this case "The sum of a number" has no definite correlation to a number, therefore, it is automatically considered a variable.
Then, since it says "The sum" we know we are adding. Adding what? We are adding an unknown term to the number 7.
Henceforth, the answer is x + 7.
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