| 4 | x | + | 3 | ( | x | − | 20 | ) | = | 8 | x | − | 7 | ( | 20 | − | x | ) |
| 方程左边 = | 4 | x | + | 3 | x | − | 3 | × | 20 |
| = | 4 | x | + | 3 | x | − | 60 |
| = | 7 | x | − | 60 |
| 7 | x | − | 60 | = | 8 | x | − | 7 | ( | 20 | − | x | ) |
| 方程右边 = | 8 | x | − | 7 | × | 20 | + | 7 | x |
| = | 8 | x | − | 140 | + | 7 | x |
| = | 15 | x | − | 140 |
| 7 | x | − | 60 | = | 15 | x | − | 140 |
| 7 | x | − | 15 | x | = | - | 140 | + | 60 |
| - | 8 | x | = | - | 140 | + | 60 |
| - | 8 | x | = | - | 80 |
| 80 | = | 8 | x |
| 8 | x | = | 80 |
| x | = | 80 | ÷ | 8 |
| = | 80 | × | 1 8 |
| = | 10 | × | 1 |
| x | = | 10 |