| ( | 20 | X | + | 300 | ) | ( | X | − | 40 | ) | = | 6080 |
| 方程左边 = | 20 | X | ( | X | − | 40 | ) | + | 300 | ( | X | − | 40 | ) |
| = | 20 | X | X | − | 20 | X | × | 40 | + | 300 | ( | X | − | 40 | ) |
| = | 20 | X | X | − | 800 | X | + | 300 | ( | X | − | 40 | ) |
| = | 20 | X | X | − | 800 | X | + | 300 | X | − | 300 | × | 40 |
| = | 20 | X | X | − | 800 | X | + | 300 | X | − | 12000 |
| = | 20 | X | X | − | 500 | X | − | 12000 |
| 20 | X | X | − | 500 | X | − | 12000 | = | 6080 |