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?

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.

Try the lab to finish this lesson.