# Variables as Unknown Numbers

## Overview

Variables can represent **unknown** numbers. We can use what we know about mathematical operations to determine the value of these unknown numbers. For example…

Determine the value of x in the expression

5x = 35

We can read the expression as **5** times what number equals **35**?

We can use the relationship between multiplication and division to solve for **x**

5x = 35

Divide both sides by 5 to get x by itself.

We can check our answer by plugging the value back into the original equation.

**5 (7) = 35 ✔**

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Determine the value of x in the expression **x – 3 = 1**

**4**

**1 + 3 = 4**

Determine the value of x in the expression **6x = 60**

**10**

Determine the value of x in the expression **2x + 3 = 9**

**3**

Determine the value of x in the expression **x ÷ 16 = 2**

**32**

**x = 2(16)**

Sarah had **15** pencils when she went to school this morning. She gave x number of pencils away. Now she has **3** pencils left. Write an equation to represent the number of pencils she gave away. Then solve for **x**.

**15 – x = 3; x = 12**

**15 – 3 = 12**

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