How to Find the Missing Number in an Equation

By Bridget Kelly
Function tables help students learn basic algebra.
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In algebra, finding the missing number in an equation is known as “solving for x.” Because we don’t know what the missing number is, we represent it using the letter “x.” The letter "x" is also known as a variable. Equations can have one or more missing variables, depending how complex they are. Finding out a missing number in a simple equation is not too difficult, though it may seem confusing at first.

Equations Involving Addition and Subtraction

Step 1

Write out the equation and represent the unknown number with the letter "x." For instance: x+3=5.

Step 2

Isolate the "x" to one side of the equation. To do that, you must get rid of the 3. Since you can’t just wipe it out, you will have to move it to the other side of the equation, which means the other side of the equal sign. When you move a number from one side of the equation to the other, you must change its sign. In the example, the 3 is positive on the left side of the equation. When you move it to the other side of the equation, it becomes negative, and the equation will look like this: x=5-3. Were this a subtraction problem (x-3=5), you would change the negative 3 to a positive when moving it the other side of the equation and the result would be: x=5+3.

Step 3

Carry out the final step; in the original equation, that would mean subtracting 3 from 5. The result would give you x=2. You have solved the equation for x and found the unknown number.

Equations Involving Multiplication and Division

Step 1

Solve the equation 2x=6 by first isolating the x to the left side of the equation. Since division is the opposite of multiplication, you will need to divide 6 by 2. The new equation will look like this: x=6/2. You now know that x=3.

Step 2

Isolate the "x" in a division problem by multiplying it by the number on the other side of the equation. Remember, division is the opposite of multiplication. Therefore, the equation x/2=6 would be rewritten as x=6(2).

Step 3

Carry out the final step; multiply 6 by 2 and you have solved the equation for x. The result is x=12. You have found the unknown number in an equation.

About the Author

Based in the American Southwest, Bridget Kelly has been writing about gardening and real estate since 2005. Her articles have appeared at Trulia.com, SFGate.com, GardenGuides.com, RE/MAX.com, MarketLeader.com, RealEstate.com, USAToday.com and in "Chicago Agent" magazine, to name a few. She holds a Bachelor of Arts in English with a concentration in creative writing.