| ( | x | − | 12 | ) | ( | 240 | + | 40 | ( | 20 | − | x | ) | ) | = | 1920 |
| 方程左边 = | x | ( | 240 | + | 40 | ( | 20 | − | x | ) | ) | − | 12 | ( | 240 | + | 40 | ( | 20 | − | x | ) | ) |
| = | x | × | 240 | + | x | × | 40 | ( | 20 | − | x | ) | − | 12 | ( | 240 | + | 40 | ( | 20 | − | x | ) | ) |
| = | 240 | x | + | x | × | 40 | × | 20 | − | x | × | 40 | x | − | 12 | ( | 240 | + | 40 | ( | 20 | − | x | ) | ) |
| = | 240 | x | + | x | × | 800 | − | x | × | 40 | x | − | 12 | ( | 240 | + | 40 | ( | 20 | − | x | ) | ) |
| = | 1040 | x | − | x | × | 40 | x | − | 12 | ( | 240 | + | 40 | ( | 20 | − | x | ) | ) |
| = | 1040 | x | − | x | × | 40 | x | − | 12 | × | 240 | − | 12 | × | 40 | ( | 20 | − | x | ) |
| = | 1040 | x | − | x | × | 40 | x | − | 2880 | − | 480 | ( | 20 | − | x | ) |
| = | 1040 | x | − | x | × | 40 | x | − | 2880 | − | 480 | × | 20 | + | 480 | x |
| = | 1040 | x | − | x | × | 40 | x | − | 2880 | − | 9600 | + | 480 | x |
| = | 1520 | x | − | x | × | 40 | x | − | 12480 |
| 1520 | x | − | x | × | 40 | x | − | 12480 | = | 1920 |