| ( | 20 | − | X | ) | × | 30 | = | ( | 30 | − | X | ) | × | 50 |
| 方程左边 = | 20 | × | 30 | − | X | × | 30 |
| = | 600 | − | X | × | 30 |
| 600 | − | 30 | X | = | ( | 30 | − | X | ) | × | 50 |
| 方程右边 = | 30 | × | 50 | − | X | × | 50 |
| = | 1500 | − | X | × | 50 |
| 600 | − | 30 | X | = | 1500 | − | 50 | X |
| - | 30 | X | + | 50 | X | = | 1500 | − | 600 |
| 20 | X | = | 1500 | − | 600 |
| 20 | X | = | 900 |
| X | = | 900 | ÷ | 20 |
| = | 900 | × | 1 20 |
| = | 45 | × | 1 |
| X | = | 45 |