| ( | 80 | − | 2 | x | ) | ( | 60 | − | 4 | x | ) | = | 4800 | × | 91 100 |
| 方程左边 = | 80 | ( | 60 | − | 4 | x | ) | − | 2 | x | ( | 60 | − | 4 | x | ) |
| = | 80 | × | 60 | − | 80 | × | 4 | x | − | 2 | x | ( | 60 | − | 4 | x | ) |
| = | 4800 | − | 320 | x | − | 2 | x | ( | 60 | − | 4 | x | ) |
| = | 4800 | − | 320 | x | − | 2 | x | × | 60 | + | 2 | x | × | 4 | x |
| = | 4800 | − | 320 | x | − | 120 | x | + | 8 | x | x |
| = | 4800 | − | 440 | x | + | 8 | x | x |
| 4800 | − | 440 | x | + | 8 | x | x | = | 4800 | × | 91 100 |
| 方程右边 = | 4368 |
| 4800 | − | 440 | x | + | 8 | x | x | = | 4368 |