| ( | 20 | X | + | 500 | ) | ( | 50 | − | X | ) | = | 30000 |
| 方程左边 = | 20 | X | ( | 50 | − | X | ) | + | 500 | ( | 50 | − | X | ) |
| = | 20 | X | × | 50 | − | 20 | X | X | + | 500 | ( | 50 | − | X | ) |
| = | 1000 | X | − | 20 | X | X | + | 500 | ( | 50 | − | X | ) |
| = | 1000 | X | − | 20 | X | X | + | 500 | × | 50 | − | 500 | X |
| = | 1000 | X | − | 20 | X | X | + | 25000 | − | 500 | X |
| = | 500 | X | − | 20 | X | X | + | 25000 |
| 500 | X | − | 20 | X | X | + | 25000 | = | 30000 |