| ( | 40 | − | x | ) | ( | 600 | + | 200 | x | ) | − | 30 | ( | 600 | + | 200 | x | ) | = | 8400 |
| 方程左边 = | 40 | ( | 600 | + | 200 | x | ) | − | x | ( | 600 | + | 200 | x | ) | − | 30 | ( | 600 | + | 200 | x | ) |
| = | 40 | × | 600 | + | 40 | × | 200 | x | − | x | ( | 600 | + | 200 | x | ) | − | 30 | ( | 600 | + | 200 | x | ) |
| = | 24000 | + | 8000 | x | − | x | ( | 600 | + | 200 | x | ) | − | 30 | ( | 600 | + | 200 | x | ) |
| = | 24000 | + | 8000 | x | − | x | × | 600 | − | x | × | 200 | x | − | 30 | ( | 600 | + | 200 | x | ) |
| = | 24000 | + | 7400 | x | − | x | × | 200 | x | − | 30 | ( | 600 | + | 200 | x | ) |
| = | 24000 | + | 7400 | x | − | x | × | 200 | x | − | 30 | × | 600 | − | 30 | × | 200 | x |
| = | 24000 | + | 7400 | x | − | x | × | 200 | x | − | 18000 | − | 6000 | x |
| = | 6000 | + | 1400 | x | − | x | × | 200 | x |
| 6000 | + | 1400 | x | − | x | × | 200 | x | = | 8400 |