| ( | x | − | 20 | ) | ( | 500 | − | 10 | x | ) | = | 1440 |
| 方程左边 = | x | ( | 500 | − | 10 | x | ) | − | 20 | ( | 500 | − | 10 | x | ) |
| = | x | × | 500 | − | x | × | 10 | x | − | 20 | ( | 500 | − | 10 | x | ) |
| = | 500 | x | − | x | × | 10 | x | − | 20 | × | 500 | + | 20 | × | 10 | x |
| = | 500 | x | − | x | × | 10 | x | − | 10000 | + | 200 | x |
| = | 700 | x | − | x | × | 10 | x | − | 10000 |
| 700 | x | − | x | × | 10 | x | − | 10000 | = | 1440 |