A variable is a letter that stands in for a number you don't know yet — it turns plain arithmetic into a puzzle-solving tool!
🔎A Variable = a Mystery Number
Instead of leaving a blank, we use a letter like x or y to hold the spot of a number we don't know yet. The very same x can be a different number in every new problem!
⚖️Expression vs. Equation
📝 EXPRESSION
A math phrase — numbers, operations, and variables.
It says "an unknown number plus five."
You can evaluate it, but there's nothing to "solve" yet.
🟰 EQUATION
A math statement: two expressions are equal.
The = sign balances both sides like a scale.
Now it's a puzzle — solve to find what x must be!
🧩Parts of an Expression
Coefficient = 3Variable = xConstant = 5
✏️Worked Example: Substitution
Evaluate x + 5 when x = 7
1
Start with the expression x + 5.
2
Substitute: swap the variable x for its value 7 → (7) + 5.
3
Add to evaluate:7 + 5 = 12.
⭐ When x = 7, x + 5 = 12
🔑Key Terms
❓VariableA letter or symbol that stands for an unknown number.
📝ExpressionA math phrase with numbers, operations & variables — but NO = sign.
🟰EquationA statement that two expressions are equal (it has an = sign).
✖️CoefficientThe number multiplied by a variable (the 3 in 3x).
🔢ConstantA number on its own — its value never changes.
🔁SubstitutionReplacing a variable with its number value to evaluate.
⭐Remember: a variable is just a letter holding the spot of a mystery number — and the moment you add an = sign, an expression becomes an equation you can solve!