What Is X?

Contributor: Meghan Vestal. Lesson ID: 11330

The "unknown" can be a scary place, especially if that unknown is the variable of an equation! No need to fear; a video and some "x"ercises will show you how to solve for "x"!


Operations and Algebraic Thinking, Pre-Algebra

learning style
personality style
Lion, Beaver
Grade Level
Intermediate (3-5)
Lesson Type
Quick Query

Lesson Plan - Get It!

  • What is x?

X marks the spot? The X-Men? The 24th letter of the alphabet?

Oh, x in an equation: 24 + x = 178.

In math, the unknown elements of an equation are called variables.

Sometimes, variables are letters and sometimes they are shapes. When you look at the problem in the beginning of the lesson, what is the variable? Share your answer with your parent or teacher.

In the beginning problem, "x" is the variable. Let's practice identifying the variable in a few more problems. Tell your teacher or parent the variable in each problem:

  • n + 7 = 21
  • ♦ + 22 = 142
  • 482 – y = 249

Did you identify what letter or symbol is the variable in each problem? As you can see, variables can be used in any type of problem. In this lesson, you will learn to determine the value of a variable in addition and subtraction problems.

Determining the value of a variable in an addition problem is very simple. You just use the opposite operation with the numbers you are given, so you can isolate the variable.

For example, the first problem is 24 + x = 178. You know x is the variable. To find out what x is equal to, use the opposite operation.

  • What is the opposite operation in this problem?

Since this is an addition problem, we need to define the opposite of addition. Tell your teacher or parent what you think is the opposite operation of addition.

Right, the opposite operation of addition is subtraction. Rearrange your problem so that you are able to subtract. The problem should now read 178 – 24 = x. Now, you are able to solve. 178 – 24 = 154, therefore, x = 154.

If you are given a subtraction problem with a variable, the steps to solve depend on what number is missing. If the unknown number is the number being subtracted, subtract the difference from the number being subtracted from.

For example, if you were solving to find y in the problem 482 – y = 249, subtract 249 (the difference) from 482 (the number being subtracted from). 482 – 249 = 233, therefore y = 233.

Sometimes the unknown number is the number being subtracted from. In these cases, add the number being subtracted to the difference.

For example, if you were solving to find x in the problem x – 15 = 45, add 15 (the number being subtracted) to 45 (the difference). 15 + 45 = 60, therefore x = 60.

For more information about how to find the value of a variable in an addition or a subtraction problem, watch this video, Algebra Basics: Solving Basic Equations Part 1 - Math Antics:


Now that you have seen a few examples, explain to your teacher or parent how to determine what a variable represents in an addition or a subtraction problem. If you are still not sure, re-watch the video.

When you can successfully explain the answer, move on to the Got It? section to practice solving problems to find the variable.

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