| 8000 | + | 7000 | + | x | = | 3 | ( | x | − | 400 | ) |
| 方程左边 = | 15000 | + | x |
| 15000 | + | x | = | 3 | ( | x | − | 400 | ) |
| 方程右边 = | 3 | x | − | 3 | × | 400 |
| = | 3 | x | − | 1200 |
| 15000 | + | x | = | 3 | x | − | 1200 |
| x | − | 3 | x | = | - | 1200 | − | 15000 |
| - | 2 | x | = | - | 1200 | − | 15000 |
| - | 2 | x | = | - | 16200 |
| 16200 | = | 2 | x |
| 2 | x | = | 16200 |
| x | = | 16200 | ÷ | 2 |
| = | 16200 | × | 1 2 |
| = | 8100 | × | 1 |
| x | = | 8100 |