| ( | x | − | 12 | ) | ( | 240 | − | 20 | ( | x | − | 20 | ) | ) | = | 1920 |
| 方程左边 = | x | ( | 240 | − | 20 | ( | x | − | 20 | ) | ) | − | 12 | ( | 240 | − | 20 | ( | x | − | 20 | ) | ) |
| = | x | × | 240 | − | x | × | 20 | ( | x | − | 20 | ) | − | 12 | ( | 240 | − | 20 | ( | x | − | 20 | ) | ) |
| = | 240 | x | − | x | × | 20 | x | + | x | × | 20 | × | 20 | − | 12 | ( | 240 | − | 20 | ( | x | − | 20 | ) | ) |
| = | 240 | x | − | x | × | 20 | x | + | x | × | 400 | − | 12 | ( | 240 | − | 20 | ( | x | − | 20 | ) | ) |
| = | 640 | x | − | x | × | 20 | x | − | 12 | ( | 240 | − | 20 | ( | x | − | 20 | ) | ) |
| = | 640 | x | − | x | × | 20 | x | − | 12 | × | 240 | + | 12 | × | 20 | ( | x | − | 20 | ) |
| = | 640 | x | − | x | × | 20 | x | − | 2880 | + | 240 | ( | x | − | 20 | ) |
| = | 640 | x | − | x | × | 20 | x | − | 2880 | + | 240 | x | − | 240 | × | 20 |
| = | 640 | x | − | x | × | 20 | x | − | 2880 | + | 240 | x | − | 4800 |
| = | 880 | x | − | x | × | 20 | x | − | 7680 |
| 880 | x | − | x | × | 20 | x | − | 7680 | = | 1920 |