Answer 1 ==
In an open sentence (an equation or inequality with a variable in it), when a number replacing the variable makes the sentence true, then it is a solution.
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The whole idea of a variable is that any number can replace it.
The term for replacing a variable with another value or expression is "substitution."
Substitution
evaluation, evaluate.
That means that you replace something, for something else. For example, a variable for a number, or a variable for some other expression.