Video: Writing and Solving Linear Inequalities

Write an inequality to describe the following problem, and then solve it: The result of subtracting −4 from a number is less than 21.

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Video Transcript

Write an inequality to describe the following problem and then solve it. The result of subtracting negative four from a number is less than 21.

What’s our starting point here? The starting point is the number we don’t know. Let’s call it 𝑥. What is happening to 𝑥? We’re getting a result of subtracting negative four from our number, subtracting negative four, the result meaning whatever we get if we subtract negative four from our number. All of this is less than 21. We say 𝑥 minus negative four is less than 21.

Now we need to solve this inequality. Saying 𝑥 minus negative four is the same thing as saying 𝑥 plus four. 𝑥 plus four is less than 21. To solve for 𝑥, subtract four from the left and the right. Plus four minus four cancels out, leaving us with 𝑥 is less than 21 minus four, which is 17. 𝑥 must be less than 17.

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