| ( | 72 | − | x | ) | ÷ | ( | 120 | − | x | ) | = | 25 100 |
| 方程两边同时乘以: | ( | 120 | − | x | ) |
| ( | 72 | − | x | ) | = | 25 100 | ( | 120 | − | x | ) |
| 72 | − | x | = | 25 100 | ( | 120 | − | x | ) |
| 72 | − | x | = | 25 100 | × | 120 | − | 25 100 | x |
| 72 | − | x | = | 30 | − | 25 100 | x |
| - | x | + | 25 100 | x | = | 30 | − | 72 |
| - | 3 4 | x | = | 30 | − | 72 |
| - | 3 4 | x | = | - | 42 |
| 42 | = | 3 4 | x |
3 4 | x | = | 42 |
| x | = | 42 | ÷ | 3 4 |
| = | 42 | × | 4 3 |
| = | 14 | × | 4 |
| x | = | 56 |