| 2100 | x | + | 2500 | ( | 50 | − | x | ) | = | 90000 |
| 方程左边 = | 2100 | x | + | 2500 | × | 50 | − | 2500 | x |
| = | 2100 | x | + | 125000 | − | 2500 | x |
| = | - | 400 | x | + | 125000 |
| - | 400 | x | + | 125000 | = | 90000 |
| - | 400 | x | = | 90000 | − | 125000 |
| - | 400 | x | = | - | 35000 |
| 35000 | = | 400 | x |
| 400 | x | = | 35000 |
| x | = | 35000 | ÷ | 400 |
| = | 35000 | × | 1 400 |
| = | 175 | × | 1 2 |
| x | = | 175 2 |
| x | = | 87.5 |