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Writing & Solving Subtraction Equations with One Variable

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  • 0:01 Writing Subtraction Equations
  • 1:07 Solving Subtraction Equations
  • 1:51 Example 1
  • 2:18 Example 2
  • 3:37 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

After watching this video lesson, you will know what is needed in order to write a subtraction equation. You will also know how to solve any subtraction equation that you are given or that you write.

Writing Subtraction Equations

In this video lesson, we talk about subtraction equations, what is needed to write them and how to solve them. Subtraction equations are those equations that use the subtraction operation. You will come across these subtraction equations in your math lessons and tests. You will also come across these in real life when you need to solve subtraction problems, such as when you are trying to figure out a discount when shopping.

So, what is needed to write a subtraction equation? You need things that are being subtracted, and then you need what they equal. Your subtraction equations will usually look something like this: x - 4 = 16. This subtraction equation could represent a problem you encounter when shopping. The x could represent the cost of the item, and the -4 can represent your discount. The 16 then represents the total you end up paying after the discount. Because the x represents the cost of the item, the problem then is that of finding the original cost of the item after a discount of four dollars has been taken.

Solving Subtraction Equations

How can you solve an equation such as x - 4 = 16? Let's take a look. Because you are trying to solve for the variable, you need to isolate the variable, to get it by itself. To do this you need to perform inverse operations to detach everything that is attached to the variable. Right now, you have a four being subtracted from the variable. The inverse operation here is addition. So, you add four to both sides of the equation. You get: x - 4 + 4 = 16 + 4. Evaluating this, you get x = 20. So, $20 was the original cost of the item before the four dollar discount. Let's take a look at a couple more examples.

Example 1

Write an equation that represents a number minus twenty equaling twelve, and then solve it.

Reading the problem, you see that the equation here is x - 20 = 12. You see that the variable has a -20 attached to it. To get the variable by itself then, you need to add the 20 to both sides of the equation. You get: x - 20 + 20 = 12 + 20. This gives you x = 32, and you are done!

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