| 450 | ÷ | ( | 120 | − | x | ) | = | 300 | ÷ | ( | 120 | − | 2 | x | ) |
| 方程两边同时乘以: | ( | 120 | − | x | ) | , | ( | 120 | − | 2 | x | ) |
| 450 | ( | 120 | − | 2 | x | ) | = | 300 | ( | 120 | − | x | ) |
| 450 | × | 120 | − | 450 | × | 2 | x | = | 300 | ( | 120 | − | x | ) |
| 450 | × | 120 | − | 450 | × | 2 | x | = | 300 | × | 120 | − | 300 | x |
| 54000 | − | 900 | x | = | 36000 | − | 300 | x |
| - | 900 | x | + | 300 | x | = | 36000 | − | 54000 |
| - | 600 | x | = | 36000 | − | 54000 |
| - | 600 | x | = | - | 18000 |
| 18000 | = | 600 | x |
| 600 | x | = | 18000 |
| x | = | 18000 | ÷ | 600 |
| = | 18000 | × | 1 600 |
| = | 30 | × | 1 |
| x | = | 30 |