| ( | 50 | + | x | − | 30 | ) | ( | 500 | − | 10 | x | ) | = | 12000 |
| 方程左边 = | 50 | ( | 500 | − | 10 | x | ) | + | x | ( | 500 | − | 10 | x | ) | − | 30 | ( | 500 | − | 10 | x | ) |
| = | 50 | × | 500 | − | 50 | × | 10 | x | + | x | ( | 500 | − | 10 | x | ) | − | 30 | ( | 500 | − | 10 | x | ) |
| = | 25000 | − | 500 | x | + | x | ( | 500 | − | 10 | x | ) | − | 30 | ( | 500 | − | 10 | x | ) |
| = | 25000 | − | 500 | x | + | x | × | 500 | − | x | × | 10 | x | − | 30 | ( | 500 | − | 10 | x | ) |
| = | 25000 | − | 0 | x | − | x | × | 10 | x | − | 30 | ( | 500 | − | 10 | x | ) |
| = | 25000 | − | x | × | 10 | x | − | 30 | × | 500 | + | 30 | × | 10 | x |
| = | 25000 | − | x | × | 10 | x | − | 15000 | + | 300 | x |
| = | 10000 | − | x | × | 10 | x | + | 300 | x |
| 10000 | − | x | × | 10 | x | + | 300 | x | = | 12000 |