| ( | 40 | − | x | ) | × | 20 | = | ( | 30 | − | x | ) | × | 30 |
| 方程左边 = | 40 | × | 20 | − | x | × | 20 |
| = | 800 | − | x | × | 20 |
| 800 | − | 20 | x | = | ( | 30 | − | x | ) | × | 30 |
| 方程右边 = | 30 | × | 30 | − | x | × | 30 |
| = | 900 | − | x | × | 30 |
| 800 | − | 20 | x | = | 900 | − | 30 | x |
| - | 20 | x | + | 30 | x | = | 900 | − | 800 |
| 10 | x | = | 900 | − | 800 |
| 10 | x | = | 100 |
| x | = | 100 | ÷ | 10 |
| = | 100 | × | 1 10 |
| = | 10 | × | 1 |
| x | = | 10 |