| 300 | ÷ | x | = | 1200 | ÷ | ( | 100 | − | x | ) |
| 方程两边同时乘以: | x | , | ( | 100 | − | x | ) |
| 300 | ( | 100 | − | x | ) | = | 1200 | x |
| 300 | × | 100 | − | 300 | x | = | 1200 | x |
| 30000 | − | 300 | x | = | 1200 | x |
| - | 300 | x | − | 1200 | x | = | - | 30000 |
| - | 1500 | x | = | - | 30000 |
| 30000 | = | 1500 | x |
| 1500 | x | = | 30000 |
| x | = | 30000 | ÷ | 1500 |
| = | 30000 | × | 1 1500 |
| = | 20 | × | 1 |
| x | = | 20 |