9 10 | × | 20 | x | + | ( | 50 | − | x | ) | × | 20 | = | 750 |
| 方程左边 = | 18 | x | + | ( | 50 | − | x | ) | × | 20 |
| 18 | x | + | ( | 50 | − | x | ) | × | 20 | = | 750 |
| 方程左边 = | 18 | x | + | 50 | × | 20 | − | x | × | 20 |
| = | 18 | x | + | 1000 | − | x | × | 20 |
| = | - | 2 | x | + | 1000 |
| - | 2 | x | + | 1000 | = | 750 |
| - | 2 | x | = | 750 | − | 1000 |
| - | 2 | x | = | - | 250 |
| 250 | = | 2 | x |
| 2 | x | = | 250 |
| x | = | 250 | ÷ | 2 |
| = | 250 | × | 1 2 |
| = | 125 | × | 1 |
| x | = | 125 |