To solve x + 5 = 12, you undo the addition by subtracting — that's a fact family idea: adding and subtracting are inverses. Warm up fact families and substituting to check, and these equations come apart easily.
Answer these 3, then press Show my path. No grade — this just points you to the right level.
1. If 4 + 5 = 9, which is also true?
2. What number makes □ + 6 = 10 true?
3. Check: does x = 3 make x + 2 = 5 true?
Addition and subtraction are inverses — they undo each other. To find a missing number, do the opposite operation. Then substitute your answer back to check.
Your quick check picks one for you, but you can switch any time:
Level 0 Fill in the missing number step by step.
A. What goes in the box? □ + 3 = 8
Undo +3 by subtracting: 8 − 3 = 5.
B. What goes in the box? □ − 2 = 4
Undo −2 by adding: 4 + 2 = 6.
C. Complete the fact family: 5 + 2 = 7, so 7 − 2 = ___
Subtracting 2 undoes adding 2: 7 − 2 = 5.
Level 1 Pick the inverse, then solve.
A. To undo + 7, what do you do to both sides?
Subtraction undoes addition — they are inverses.
B. Solve: x + 7 = 12. What is x?
12 − 7 = 5. Check: 5 + 7 = 12. ✓
Level 2 Solve and check by substituting.
A. Solve: x − 4 = 6. What is x?
Undo −4 by adding: 6 + 4 = 10. Check: 10 − 4 = 6. ✓
B. Solve: 9 = x + 2. What is x?
9 − 2 = 7. Check: 7 + 2 = 9. ✓
1. Solve: x + 6 = 11. What is x?
11 − 6 = 5. Check: 5 + 6 = 11. ✓
2. Which step solves x − 3 = 5?
Adding 3 undoes subtracting 3: x = 5 + 3 = 8.
You've practiced exactly what Lesson 7-2 uses. Time to dive in.
Start Lesson 7-2 →