| 2250 | = | ( | 500 | − | 10 | x | ) | ( | x | − | 20 | ) |
| 方程右边 = | 500 | ( | x | − | 20 | ) | − | 10 | x | ( | x | − | 20 | ) |
| = | 500 | x | − | 500 | × | 20 | − | 10 | x | ( | x | − | 20 | ) |
| = | 500 | x | − | 10000 | − | 10 | x | ( | x | − | 20 | ) |
| = | 500 | x | − | 10000 | − | 10 | x | x | + | 10 | x | × | 20 |
| = | 500 | x | − | 10000 | − | 10 | x | x | + | 200 | x |
| = | 700 | x | − | 10000 | − | 10 | x | x |
| 2250 | = | 700 | x | − | 10000 | − | 10 | x | x |