Skill: Represent a problem with an equation

An equation shows how the quantities in a problem are related. First, decide what the unknown represents. Use a letter, such as $s$, for that unknown.

Example:
Mia had some stickers. She gave away $7$ stickers and now has $15$ stickers. How many stickers did she have at the start?

1. Name the unknown.
   The starting number of stickers is unknown, so let $s$ represent it.

2. Describe what happened.
   Mia started with $s$ stickers and gave away $7$. Giving away means subtracting:

$$s-7$$

3. Show what remains.
   She has $15$ stickers now. The expression $s-7$ and the number $15$ have the same value, so use an equals sign:

$$s-7=15$$

4. Solve and check.
   Add $7$ to both sides:

$$s=22$$

Check the situation:

$$22-7=15$$

Mia had $22$ stickers at the start. The equation matches the story because it shows the starting amount, the change, and the amount left.