Solve one-step equations · lesson

Undo adding or subtracting

💡 Do the opposite operation to both sides: subtract to undo adding, add to undo subtracting.

An equation is like a balance. Whatever you do to one side, do to the other.

If a number is added to x, subtract it from both sides. If a number is subtracted from x, add it to both sides.

Check by putting your answer back in.

🧠 Let's work it out

Solve x − 6 = 15.

What do you think comes next?

equation: a math sentence with an equals sign
solve: find the value of the letter that makes the equation true
inverse operation: the operation that undoes another, like subtract undoes add

💡 Divide to undo multiplying, and multiply to undo dividing.

6x means 6 times x. To undo times 6, divide both sides by 6.

x ÷ 3 means x divided by 3. To undo it, multiply both sides by 3.

Decimals work the same way.

🧠 Let's work it out

Solve 6x = 48.

What do you think comes next?

💡 Let a letter stand for the unknown, write the equation the story describes, then solve.

Pick a letter for the number you don't know. Write what the story says happened to it.

"Gave away 5 and has 18 left" means the start minus 5 equals 18.

🧠 Let's work it out

Leo gave away 5 cards. Now he has 18. How many did he have?

What do you think comes next?

💡 To undo times a fraction, multiply by its reciprocal; to undo minus a fraction, add it.

(3/4)x means 3/4 times x. To undo it, multiply both sides by the flip, 4/3.

If a fraction is subtracted from x, add it back. Use a common bottom number to add.

🧠 Let's work it out

Solve (3/4)x = 9.

What do you think comes next?