| ( | x | − | 40 | ) | × | 30 | ( | 60 | − | x | ) | = | 6480 |
| 方程左边 = | x | × | 30 | ( | 60 | − | x | ) | − | 40 | × | 30 | ( | 60 | − | x | ) |
| = | x | × | 30 | ( | 60 | − | x | ) | − | 1200 | ( | 60 | − | x | ) |
| = | x | × | 30 | × | 60 | − | x | × | 30 | x | − | 1200 | ( | 60 | − | x | ) |
| = | x | × | 1800 | − | x | × | 30 | x | − | 1200 | ( | 60 | − | x | ) |
| = | 1800 | x | − | x | × | 30 | x | − | 1200 | × | 60 | + | 1200 | x |
| = | 1800 | x | − | x | × | 30 | x | − | 72000 | + | 1200 | x |
| = | 3000 | x | − | x | × | 30 | x | − | 72000 |
| 3000 | x | − | x | × | 30 | x | − | 72000 | = | 6480 |