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All courses Math Foundations · MF.EQ.9 36 of 60
Write and solve a linear equation from a short verbal or contextual situation.

Write and solve an additive word-problem equation

Problem
Mia got \(7~\text{more}~\text{stickers}\) and then had \(18\). Write and solve an equation for her starting number of stickers.
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Hint

Let a variable stand for the number of stickers Mia had at first.

This is a start + change = total situation. She got 7 more, and 18 is the final total.

Solution walkthrough

01

Define the starting amount and translate

\[\text{let}~x~=~\text{starting}~\text{stickers};~x~+~7~=~18\]

Mia's unknown start increases by 7 to reach the final total 18.

02

Undo the increase

\[x~+~7~-~7~=~18~-~7\]

Subtracting 7 from both sides isolates the starting amount.

03

Compute and check

\[x~=~11;~11~+~7~=~18\]

Eleven stickers plus the seven she got equals the stated total.

04

Report equation and solution

\[x~+~7~=~18;~x~=~11\]

The equation models the story and the solution is Mia's starting count.

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Another way

  1. Work backward from 18 by removing the 7 new stickers: 18 - 7 = 11.

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Common mistake

Do not use x - 7 = 18. The story says Mia got more stickers, so the change from start to finish is addition.

Solution walkthrough video