| ( | 80 | − | x | ) | ( | 200 | + | 10 | x | ) | − | 50 | × | 800 | = | 9000 |
| 方程左边 = | ( | 80 | − | x | ) | ( | 200 | + | 10 | x | ) | − | 40000 |
| ( | 80 | − | x | ) | ( | 200 | + | 10 | x | ) | − | 40000 | = | 9000 |
| 方程左边 = | 80 | ( | 200 | + | 10 | x | ) | − | x | ( | 200 | + | 10 | x | ) | − | 40000 |
| = | 80 | × | 200 | + | 80 | × | 10 | x | − | x | ( | 200 | + | 10 | x | ) | − | 40000 |
| = | 16000 | + | 800 | x | − | x | ( | 200 | + | 10 | x | ) | − | 40000 |
| = | - | 24000 | + | 800 | x | − | x | ( | 200 | + | 10 | x | ) |
| = | - | 24000 | + | 800 | x | − | x | × | 200 | − | x | × | 10 | x |
| = | - | 24000 | + | 600 | x | − | x | × | 10 | x |
| - | 24000 | + | 600 | x | − | x | × | 10 | x | = | 9000 |