Put it back in
Four answers were found for 3x + 6 = 18: 8, 4, 0 and 6. Only one can fit. An answer fits when both sides then match. Which one?
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x = 4 Step 1 of 4
- Answer:
x = 4 - Put it into
3x + 6. 3 × 4 + 6 = 1818 = 18— it fits.
Four ways, four answers, one test
Only 4 fits. The other three do not.
- 4: take 6 off both sides, then share by 3.
- 0: share only the 18 by 3, then take 6 off.
- 6: take 6 off the left side only.
- 8: put the 6 on the right side as +6.
Test each in 3x + 6 = 18. Each time, work out 3 × the number, then add 6. At x = 4 both sides are 18. At x = 0 the left side is 6. At x = 6 the left side is 24. At x = 8 the left side is 30.
Only one fits. The check works however you found the number.
Rule: Put the answer back into the first equation. Then both sides must match.