| 1200 | ( | 400 | + | x | ) | = | 1300 | ( | 400 | + | x | ) |
| 方程左边 = | 1200 | × | 400 | + | 1200 | x |
| = | 480000 | + | 1200 | x |
| 480000 | + | 1200 | x | = | 1300 | ( | 400 | + | x | ) |
| 方程右边 = | 1300 | × | 400 | + | 1300 | x |
| = | 520000 | + | 1300 | x |
| 480000 | + | 1200 | x | = | 520000 | + | 1300 | x |
| 1200 | x | − | 1300 | x | = | 520000 | − | 480000 |
| - | 100 | x | = | 520000 | − | 480000 |
| - | 100 | x | = | 40000 |
| - | 40000 | = | 100 | x |
| 100 | x | = | - | 40000 |
| x | = | - | 40000 | ÷ | 100 |
| = | - | 40000 | × | 1 100 |
| = | - | 400 | × | 1 |
| x | = | - | 400 |