| ( | 400 | − | 40 | x | ) | ( | 20 | + | 8 | x | ) | = | 12000 |
| 方程左边 = | 400 | ( | 20 | + | 8 | x | ) | − | 40 | x | ( | 20 | + | 8 | x | ) |
| = | 400 | × | 20 | + | 400 | × | 8 | x | − | 40 | x | ( | 20 | + | 8 | x | ) |
| = | 8000 | + | 3200 | x | − | 40 | x | ( | 20 | + | 8 | x | ) |
| = | 8000 | + | 3200 | x | − | 40 | x | × | 20 | − | 40 | x | × | 8 | x |
| = | 8000 | + | 3200 | x | − | 800 | x | − | 320 | x | x |
| = | 8000 | + | 2400 | x | − | 320 | x | x |
| 8000 | + | 2400 | x | − | 320 | x | x | = | 12000 |
| x1= | 5 2 |
| x2=5 |