| ( | 400 | + | 20 | x | ) | ( | 15 | − | x | ) | = | 4250 |
| 方程左边 = | 400 | ( | 15 | − | x | ) | + | 20 | x | ( | 15 | − | x | ) |
| = | 400 | × | 15 | − | 400 | x | + | 20 | x | ( | 15 | − | x | ) |
| = | 6000 | − | 400 | x | + | 20 | x | ( | 15 | − | x | ) |
| = | 6000 | − | 400 | x | + | 20 | x | × | 15 | − | 20 | x | x |
| = | 6000 | − | 400 | x | + | 300 | x | − | 20 | x | x |
| = | 6000 | − | 100 | x | − | 20 | x | x |
| 6000 | − | 100 | x | − | 20 | x | x | = | 4250 |