| ( | 300 | + | 20 | x | ) | ( | 60 | − | x | ) | = | 6080 |
| 方程左边 = | 300 | ( | 60 | − | x | ) | + | 20 | x | ( | 60 | − | x | ) |
| = | 300 | × | 60 | − | 300 | x | + | 20 | x | ( | 60 | − | x | ) |
| = | 18000 | − | 300 | x | + | 20 | x | ( | 60 | − | x | ) |
| = | 18000 | − | 300 | x | + | 20 | x | × | 60 | − | 20 | x | x |
| = | 18000 | − | 300 | x | + | 1200 | x | − | 20 | x | x |
| = | 18000 | + | 900 | x | − | 20 | x | x |
| 18000 | + | 900 | x | − | 20 | x | x | = | 6080 |