| ( | 2 | × | 20 | − | X | ) | × | 15 | = | ( | 3 | × | 20 | − | X | ) | × | 9 |
| 方程左边 = | 2 | × | 20 | × | 15 | − | X | × | 15 |
| = | 600 | − | X | × | 15 |
| 600 | − | 15 | X | = | ( | 3 | × | 20 | − | X | ) | × | 9 |
| 方程右边 = | 3 | × | 20 | × | 9 | − | X | × | 9 |
| = | 540 | − | X | × | 9 |
| 600 | − | 15 | X | = | 540 | − | 9 | X |
| - | 15 | X | + | 9 | X | = | 540 | − | 600 |
| - | 6 | X | = | 540 | − | 600 |
| - | 6 | X | = | - | 60 |
| 60 | = | 6 | X |
| 6 | X | = | 60 |
| X | = | 60 | ÷ | 6 |
| = | 60 | × | 1 6 |
| = | 10 | × | 1 |
| X | = | 10 |