| ( | x | − | 400 | ) | ( | 300 | + | 1200 | − | 20 | x | ) | = | 6080 |
| 方程左边 = | x | ( | 300 | + | 1200 | − | 20 | x | ) | − | 400 | ( | 300 | + | 1200 | − | 20 | x | ) |
| = | x | × | 300 | + | x | × | 1200 | − | x | × | 20 | x | − | 400 | ( | 300 | + | 1200 | − | 20 | x | ) |
| = | 1500 | x | − | x | × | 20 | x | − | 400 | ( | 300 | + | 1200 | − | 20 | x | ) |
| = | 1500 | x | − | x | × | 20 | x | − | 400 | × | 300 | − | 400 | × | 1200 | + | 400 | × | 20 | x |
| = | 1500 | x | − | x | × | 20 | x | − | 120000 | − | 480000 | + | 8000 | x |
| = | 9500 | x | − | x | × | 20 | x | − | 600000 |
| 9500 | x | − | x | × | 20 | x | − | 600000 | = | 6080 |