Lesson 6 of 16
Two steps, undone backwards
Show on a big screenYou think of a number. You triple it, which means times 3. Then you add 6. You get 18. What was the number?
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3x + 6 = 18 Step 1 of 4
3x + 6 = 18- Added last. Comes off first.
3x = 12- Share by 3:
x = 4.
Undo the last step first
The number was 4. Call the number x. Then 3x means 3 times x.
Undo steps in the opposite order to how they were done.
The number was tripled, then 6 was added. So take the 6 off first.
- Start at 18.
- Take away 6. That leaves 12.
- Share by 3. That leaves 4.
Check it forward: 3 × 4 = 12, and 12 + 6 = 18.
Rule: Undo the last step first.
Take 4x − 9 = 27. Add 9 to both sides: 4x = 36. Share by 4: x = 9. Here 27 + 9 = 36 and 36 ÷ 4 = 9. Sharing first leaves 9 ÷ 4 and 27 ÷ 4. Those are not whole numbers.
Watch out!
Share first? Then share every term: 3x, 6 and 18. 3x + 6 = 18 becomes x + 2 = 6, not x + 6 = 6.
Try the lab to finish this lesson.