| 480 | = | ( | x | − | 20 | ) | ( | - | 5 | x | + | 200 | ) |
| 方程右边 = | x | ( | - | 5 | x | + | 200 | ) | − | 20 | ( | - | 5 | x | + | 200 | ) |
| = | - | x | × | 5 | x | + | x | × | 200 | − | 20 | ( | - | 5 | x | + | 200 | ) |
| = | - | x | × | 5 | x | + | 200 | x | + | 20 | × | 5 | x | − | 20 | × | 200 |
| = | - | x | × | 5 | x | + | 200 | x | + | 100 | x | − | 4000 |
| = | - | x | × | 5 | x | + | 300 | x | − | 4000 |
| 480 | = | - | x | × | 5 | x | + | 300 | x | − | 4000 |