x 3 x + 3 = 8 Come play with numbers!
One-step equations

How do we unwrap a + without breaking equality?

# Undo + and − Addition and subtraction are **opposites**. To undo `+ 6`, do `− 6`. To undo `− 4`, do `+ 4`. ## Example `x + 9 = 20` Undo +9 → subtract 9: `x = 11` ## Example `x − 7 = 3` Undo −7 → add 7: `x = 10` ## Check `11 + 9 = 20` ✓ `10 − 7 = 3` ✓ Always put your answer back. It takes ten seconds and catches slips. ## One move, then a check Every one-step “plus or minus” equation is the same dance: 1. What is stuck to x? 2. Do the opposite to **both** sides. 3. Read x. 4. Cover x with that number in the **original** line. ### Opposite table | Stuck to x | Do this to both sides | | --- | --- | | + 6 | − 6 | | − 4 | + 4 | | + 15 | − 15 | ### A special pair `x + 12 = 12` → x = 0. A box plus 12 makes 12, so the box is empty (zero). `x − 8 = 0` → x = 8. A box minus 8 makes nothing, so the box held 8.

By the end of this lesson

Solve one-step equations that undo + or −, then substitute back to check.

Remember

  • Addition and subtraction are opposites.
  • To undo + a, subtract a from both sides.
  • To undo − a, add a to both sides.
  • Always put the answer back into the original equation.
  • Write: original → one balanced move → x = … → check.

Worked examples

Cover the answer. Try the steps on paper, then open them.

Example 1 — Undo plus

x + 9 = 20

  1. +9 is glued to x.
  2. Subtract 9 from both sides.
  3. x = 11
  4. Check: 11 + 9 = 20 ✓

Answer: x = 11

Check: 20 − 9 = 11.

Example 2 — Undo minus

x − 7 = 3

  1. −7 is glued to x.
  2. Add 7 to both sides.
  3. x = 10
  4. Check: 10 − 7 = 3 ✓

Answer: x = 10

Check: 3 + 7 = 10.

Example 3 — Bigger numbers

x + 18 = 45

  1. Subtract 18 from both sides.
  2. 45 − 18 = 27
  3. Check: 27 + 18 = 45 ✓

Answer: x = 27

Check: Same method, larger arithmetic.

Common mistakes

Slip x + 9 = 20 → x = 20 + 9 = 29

Fix You must subtract 9, not add it again.

Undo means the opposite.

Slip x − 7 = 3 → x = 3 − 7 = −4

Fix Undo minus by adding: 3 + 7 = 10.

The 7 was subtracted from x, so give it back.

Slip Forgetting to check.

Fix 11 + 9 = 20 takes ten seconds and catches slips.

Checking is part of the method.

Try on paper first

Write the answer, then tap Reveal. These are not the quiz — they are warm-ups.

1. x + 6 = 15 → x =

Answer: 9

2. x − 4 = 10 → x =

Answer: 14

3. x + 12 = 12 → x =

Answer: 0

4. x − 8 = 0 → x =

Answer: 8

Touch practice

8 tap / type

Tap an option or type the answer. Retry any miss. Extra generated questions appear after the set if this topic allows it.

Optional picture lab

After practice

Real world

Undo putting 5 sweets into a box.

Connect

Inverse operations in algorithms.

⭐ Star quiz

9 questions

Finish the touch practice first if you can. Then answer without peeking. Read every “why”.

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