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3rd-5th Grade Math: Practice & Review37 chapters | 252 lessons

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Lesson Transcript

Instructor:
*Ashley Donohoe*

Watch this video lesson to learn how you can find the missing value in a subtraction equation. Learn what it takes to make sure that both sides of a subtraction equation are equal.

In this lesson, we will talk about subtraction equations. A **subtraction equation** is a mathematical equation that involves the subtraction operator. Specifically, we will talk about those mathematical equations that show two numbers being subtracted being equal two other numbers being subtracted. Because there is an equal sign in between, both sides must equal the same number. For example, 5 - 3 = 10 - 8 is a subtraction equation in which both sides equal the same thing. 5 - 3 = 2 and 10 - 8 = 2.

It's important to know how to work with these subtraction equations because being able to spot whether both sides of an equation are equal or not is a good skill to have. For example, you may need to use this skill when taking tests and when figuring out problems in the real world.

When you are taking a test or figuring out a problem in the real world, you won't always be given all the numbers like in the first example. You won't see 5 - 3 = 10 - 8. Instead, you might see 5 - ? = 10 - 8 or even 5 - 3 = ? - 8.

The missing value could be anywhere in your subtraction equation. It is now your job to find the missing value. We call finding the missing value in a subtraction equation balancing our subtraction equation. It's called balancing because after we are done, both sides will be equal.

If you think about two things being weighed on one of those old-time scales where you have two bowls on either side, then you can see how the subtraction equation is balanced once both sides are equal. So, how do you go about balancing a subtraction equation?

Let's use our critical thinking skills to see if we can figure this thing out. Let's say we need to balance this equation:

*45 - ? = 90 - 50*

Thinking about it, we know that both sides must equal the same thing. So, it would help us to know what both sides must equal. Looking at the subtraction equation, we see that we have a full subtraction expression on the right side of our equal sign. We can go ahead and subtract those two numbers to find what both sides must equal. We get 90 - 50 = 40. So this tells us that the left side must also equal 40. So, we think, 'What number subtracted from 45 will give us 40: 45 - ? = 40?' If we start counting down from 45, how many numbers do we need to count to get to 40? We count 5. So, we need to subtract 5 from 45 to get 40. Our missing value is 5.

From working out this problem, we can create the following steps to balance subtraction problems. The first step is to complete the subtraction on the side without the missing value. This will give us the number that the other side must equal. We will then use this number to find what missing value will give us that number.

Let's look at another example.

*30 - 11 = ? - 3*

Looking at this problem, we see that we can go ahead and complete the subtraction on the left side of the equal sign. We get 30 - 11 = 19. This tells us that the right side must also equal 19. So, what number minus 3 will give us 19: ? - 3 = 19? We know our missing value must be higher than 19. Specifically, it is 3 higher than 19. So, we can actually count up 3 numbers from 19. What do we get? We get 19 + 3 = 22. Our missing value is 22.

Let's review what we've learned.

A **subtraction equation** is a mathematical equation that involves the subtraction operator. We call finding the missing value in a subtraction equation balancing our subtraction equation. The steps to balancing subtraction problems are:

1) Complete the subtraction on the side without the missing value

2) Use this number to find what missing value will give us that number.

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