| - | 4200 | ÷ | x | + | 120 | = | 2400 | ÷ | ( | x | − | 30 | ) |
| 方程两边同时乘以: | x | , | ( | x | − | 30 | ) |
| - | 4200 | ( | x | − | 30 | ) | + | 120 | x | ( | x | − | 30 | ) | = | 2400 | x |
| - | 4200 | x | + | 4200 | × | 30 | + | 120 | x | ( | x | − | 30 | ) | = | 2400 | x |
| - | 4200 | x | + | 126000 | + | 120 | x | ( | x | − | 30 | ) | = | 2400 | x |
| - | 4200 | x | + | 126000 | + | 120 | x | x | − | 120 | x | × | 30 | = | 2400 | x |
| - | 4200 | x | + | 126000 | + | 120 | x | x | − | 3600 | x | = | 2400 | x |
| - | 7800 | x | + | 126000 | + | 120 | x | x | = | 2400 | x |