| ( | x | − | 15 | ) | ( | 1080 | − | 40 | x | ) | = | 1280 |
| 方程左边 = | x | ( | 1080 | − | 40 | x | ) | − | 15 | ( | 1080 | − | 40 | x | ) |
| = | x | × | 1080 | − | x | × | 40 | x | − | 15 | ( | 1080 | − | 40 | x | ) |
| = | 1080 | x | − | x | × | 40 | x | − | 15 | × | 1080 | + | 15 | × | 40 | x |
| = | 1080 | x | − | x | × | 40 | x | − | 16200 | + | 600 | x |
| = | 1680 | x | − | x | × | 40 | x | − | 16200 |
| 1680 | x | − | x | × | 40 | x | − | 16200 | = | 1280 |