Lesson 3: Write and Solve Addition and Subtraction Equations

Property

Subtraction Property of Equality
For all real numbers $a$, $b$, and $c$, if $a = b$, then $a - c = b - c$.

Addition Property of Equality
For all real numbers $a$, $b$, and $c$, if $a = b$, then $a + c = b + c$.
When you add or subtract the same quantity from both sides of an equation, you still have equality.

Examples

• To solve $p - 7 = 12$, we use the Addition Property. Add 7 to both sides: $p - 7 + 7 = 12 + 7$, which simplifies to $p = 19$.

Property

Use variables to represent unknown quantities in word problems and construct simple equations that can be solved using addition or subtraction. This involves identifying the unknown, assigning it a variable, and translating the problem into an equation of the form $x + a = b$ or $x - a = b$.

Examples