| ( | 600 | − | 20 | X | ) | ( | X | + | 1 | ) | = | 2600 |
| 方程左边 = | 600 | ( | X | + | 1 | ) | − | 20 | X | ( | X | + | 1 | ) |
| = | 600 | X | + | 600 | × | 1 | − | 20 | X | ( | X | + | 1 | ) |
| = | 600 | X | + | 600 | − | 20 | X | ( | X | + | 1 | ) |
| = | 600 | X | + | 600 | − | 20 | X | X | − | 20 | X | × | 1 |
| = | 600 | X | + | 600 | − | 20 | X | X | − | 20 | X |
| = | 580 | X | + | 600 | − | 20 | X | X |
| 580 | X | + | 600 | − | 20 | X | X | = | 2600 |