| ( | 20 | ( | 60 | − | x | ) | + | 300 | ) | x | − | 40 | × | 300 | = | 6080 |
| 方程左边 = | ( | 20 | ( | 60 | − | x | ) | + | 300 | ) | x | − | 12000 |
| ( | 20 | ( | 60 | − | x | ) | + | 300 | ) | x | − | 12000 | = | 6080 |
| 方程左边 = | 20 | ( | 60 | − | x | ) | x | + | 300 | x | − | 12000 |
| = | 20 | × | 60 | x | − | 20 | x | x | + | 300 | x | − | 12000 |
| = | 1200 | x | − | 20 | x | x | + | 300 | x | − | 12000 |
| = | 1500 | x | − | 20 | x | x | − | 12000 |
| 1500 | x | − | 20 | x | x | − | 12000 | = | 6080 |