# Problem-Solving using Linear Equations

Lesson Transcript
Instructor: Jeff Calareso

Jeff teaches high school English, math and other subjects. He has a master's degree in writing and literature.

From sale prices to trip distances, many real life problems can be solved using linear equations. In this lesson, we'll practice translating word problems into linear equations, then solving the problems.

## Real World Math

A train leaves Chicago at 7 a.m., traveling at 70 mph to New York, which is 800 miles away. Another train leaves New York at the same time, traveling on a parallel track to Chicago at 85 mph. When will they meet? The question is: why do we care so much about trains?

Well, I like trains, but I still feel a little nervous when I read a math problem that starts with a train. If I'm going to have to translate a real world scenario to an algebraic equation, can't it be something I might actually encounter in my life? I mean, I've ridden trains between Chicago and New York, but I've never plotted when my train will pass another.

In this lesson, we'll not only practice solving problems that can be translated into linear equations, we'll also focus on problems you may encounter in your life - problems not involving trains passing each other. An error occurred trying to load this video.

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Coming up next: Solving Linear Equations with Literal Coefficients

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• 0:05 Real-World Math
• 0:53 Linear Equations
• 1:11 Simple Problems
• 2:02 Practice Problems
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## Linear Equations

As a reminder, a linear equation is just an algebraic expression that represents a line. These equations typically have one variable and look like 3x = 9 or y + 4 = 10. In these equations, we're trying to figure out the variable, which involves getting it alone on one side of the equals sign.

## Simple Problems

There are simple problems that involve linear equations. For example, the sum of 35 and a number is 72. What is the number?

The thing we don't know is our variable. Let's use x here. We know x + 35 = 72, so that's our equation. If we solve for x by subtracting 35 from both sides, we get x = 37. Now we know our number.

They can be a bit more complex, like this: 15 less than four times a number is 57. What is the number? Again, let's use x for the number. Four times that number is 4x. 15 less than that is 4x -15. So, our equation is 4x - 15 = 57. To solve for x, we add 15 to both sides. Then, we divide by 4, and x = 18.

## Practice Problems

Let's take that knowledge and look at some real life situations. Let's start with money. We all like money, right?

Let's say you're a little short on cash and need a loan. Your cousin agrees to loan you money, and you agree that you'll repay him in full plus 4% interest. We're going to ignore the questionable judgment he displays in loaning money to family. If he loans you \$500, how much interest will you need to pay?

Our variable here is the amount of interest, so let's call that x. The interest will be the amount of the loan, \$500, multiplied by the interest rate, 4%. To multiply with a percent, we convert it to a decimal. So, x = 500*.04. What is 500 * .04? 20. So, you'll owe him \$20, plus the original \$500.

You decide that you want to be better about saving money. Your cell phone company is promoting a text message plan that costs \$10 each month plus five cents per text. You currently pay \$20 each month for an unlimited plan, but you want to save a few dollars. If you want to try the new plan and spend only \$15 each month, how many texts can you send?

Ok, let's use t for texts. It costs five cents per text, so that's .05t. Plus, there's that \$10 fee. So, you want .05t + 10 to equal 15. First, subtract 10 from both sides. Then, divide by .05. So, you could send 100 texts each month. You've been averaging way more than that, so maybe this isn't a great plan.

But, then you get a new job, and suddenly you have some extra cash. You decide that you want to save up for new bike. You find one you like that costs \$400. If you can save \$35 each week, how many weeks will it take you to get the bike?

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