A two-step equation like 3x + 5 = 20 does two things to x: it multiplies by 3, then adds 5. To solve it, undo those steps in reverse order. First subtract 5 from both sides: 3x = 15. Then divide both sides by 3: x = 5. Check by substituting: 3 × 5 + 5 = 20. ✓ The order matters — take the added number off first, then divide. Stopping after the subtraction (giving 15) or dividing before subtracting are the two most common mistakes.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- coefficient
- the number multiplying the letter (the 3 in 3x)
- constant
- the plain number added on (the 5 in 3x + 5)
- inverse
- the opposite operation that undoes another
Worked examples
3x + 5 = 20
- 1.Subtract 5: 3x = 15.
- 2.Divide by 3: x = 5.
= 5
2x + 6 = 16
- 1.Subtract 6: 2x = 10.
- 2.Divide by 2: x = 5.
= 5
4x + 8 = 28
- 1.Subtract 8: 4x = 20.
- 2.Divide by 4: x = 5.
= 5
For parents
Method: Subtract first, then divide.
What to say: Ask 'what was done last to x?' — undo that first, then work back.
Common mistakes:
- Stopping after subtracting and forgetting to divide (3x + 5 = 20 → 15).
- Dividing before subtracting the constant.