Back To Course

6th-8th Grade Math: Practice & Review55 chapters | 469 lessons

Are you a student or a teacher?

Try Study.com, risk-free

As a member, you'll also get unlimited access to over 75,000 lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed.

Try it risk-freeWhat teachers are saying about Study.com

Already registered? Login here for access

Your next lesson will play in
10 seconds

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.

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.

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.

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!

Write an equation that represents thirty-one minus a number equaling nineteen, and then solve it.

Reading this problem, you see that the equation here is 31 - *x* = 19. Notice here that the variable is in a different location. Notice also that the variable is now the one being subtracted. The other number, the 31, is positive. So, how do you solve this one?

We still keep performing inverse operations to solve. First, we need to detach the number 31. To do this, we need to subtract the 31 from both sides. Why subtraction? Because the 31 is positive, it is actually being added to the variable. In this case, the 31 is being added to a negative variable: 31 - *x* - 31 = 19 - 31. Evaluating this, you get -*x* = -12.

Now you need to detach the negative sign from the variable. To do this, you divide by a -1. You get -*x* / -1 = -12 / -1. Evaluating this, you get *x* = 12. Dividing a negative number by a negative one turns it into a positive number. The negative signs cancel each other out. So, our answer is 12.

Let's review what we've learned. **Subtraction equations** are those equations that use the subtraction operation. A complete subtraction equation will have the things being subtracted on one side and what they equal on the other side of the equal sign. To solve these subtraction equations, you keep performing inverse operations until you have isolated the variable.

When you are finished here, you should be able to write and solve a subtraction equation that has one unknown variable.

To unlock this lesson you must be a Study.com Member.

Create your account

Are you a student or a teacher?

Already a member? Log In

BackWhat teachers are saying about Study.com

Already registered? Login here for access

Did you know… We have over 160 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

You are viewing lesson
Lesson
4 in chapter 24 of the course:

Back To Course

6th-8th Grade Math: Practice & Review55 chapters | 469 lessons

- Inverse Operations in Math: Definition & Examples 4:50
- Writing & Solving Addition Equations with One Variable 5:48
- Writing & Solving Addition Word Problems with One Variable 5:25
- Writing & Solving Subtraction Equations with One Variable 4:06
- Writing & Solving Multiplication Equations with One Variable 5:08
- Writing & Solving Multiplication Word Problems with One Variable 4:48
- Writing & Solving Division Equations with One Variable 5:06
- Writing & Solving Division Word Problems with One Variable 4:44
- How to Use Algebra Tiles to Model & Solve Equations 6:12
- Go to 6th-8th Grade Algebra: Writing & Solving One-Step Equations

- Computer Science 335: Mobile Forensics
- Electricity, Physics & Engineering Lesson Plans
- Teaching Economics Lesson Plans
- U.S. Politics & Civics Lesson Plans
- US History - Civil War: Lesson Plans & Resources
- iOS Data Analysis & Recovery
- Acquiring Data from iOS Devices
- Foundations of Digital Forensics
- Introduction to Mobile Forensics
- Examination of iOS Devices
- CNE Prep Product Comparison
- IAAP CAP Prep Product Comparison
- TACHS Prep Product Comparison
- Top 50 Blended Learning High Schools
- EPPP Prep Product Comparison
- NMTA Prep Product Comparison
- Study.com NMTA Scholarship: Application Form & Information

- History of Sparta
- Realistic vs Optimistic Thinking
- How Language Reflects Culture & Affects Meaning
- Logical Thinking & Reasoning Questions: Lesson for Kids
- Human Geography Project Ideas
- Asian Heritage Month Activities
- Types of Visualization in Python
- Quiz & Worksheet - Frontalis Muscle
- Octopus Diet: Quiz & Worksheet for Kids
- Quiz & Worksheet - Fezziwig in A Christmas Carol
- Quiz & Worksheet - Dolphin Mating & Reproduction
- Flashcards - Measurement & Experimental Design
- Flashcards - Stars & Celestial Bodies
- Narrative Essay Topics for Teachers
- High School Science Worksheets and Printables

- CLEP Biology: Study Guide & Test Prep
- CLEP American Literature: Study Guide & Test Prep
- Algebra I: Credit Recovery
- Introduction to Psychology: Homework Help Resource
- Middle School Earth Science Curriculum Resource & Lesson Plans
- Holt United States History Chapter 11: Expanding West (1800-1855)
- AP World History - Major Belief Systems: Homework Help
- Quiz & Worksheet - Overview of the Schlieffen Plan in WW1
- Quiz & Worksheet - Approaches & Procedures of Research Methodology
- Quiz & Worksheet - How to Teach Children Listening Skills
- Quiz & Worksheet - Optic Nerve Damage
- Quiz & Worksheet - Characteristics of Genotypic Variation

- Intrapersonal Learning Style: Teaching Tips
- The Difference Between Trypsin & Chymotrypsin
- Reading Games for Kids
- How to Stop Procrastinating
- South Dakota State Standards for Math
- Texas Teacher Certification Renewal
- Fractions Lesson Plan
- Minnesota Science Standards for 3rd Grade
- Narrative Writing Rubric Examples
- How to Learn Accounting
- Cognitive Learning Activities for the Classroom
- Engineering Summer Programs for High School Students

- Tech and Engineering - Videos
- Tech and Engineering - Quizzes
- Tech and Engineering - Questions & Answers

Browse by subject