| ( | 50 | + | X | ) | ( | 500 | − | 10 | X | ) | − | 40 | ( | 500 | − | 10 | X | ) | = | 8000 |
| 方程左边 = | 50 | ( | 500 | − | 10 | X | ) | + | X | ( | 500 | − | 10 | X | ) | − | 40 | ( | 500 | − | 10 | X | ) |
| = | 50 | × | 500 | − | 50 | × | 10 | X | + | X | ( | 500 | − | 10 | X | ) | − | 40 | ( | 500 | − | 10 | X | ) |
| = | 25000 | − | 500 | X | + | X | ( | 500 | − | 10 | X | ) | − | 40 | ( | 500 | − | 10 | X | ) |
| = | 25000 | − | 500 | X | + | X | × | 500 | − | X | × | 10 | X | − | 40 | ( | 500 | − | 10 | X | ) |
| = | 25000 | − | 0 | X | − | X | × | 10 | X | − | 40 | ( | 500 | − | 10 | X | ) |
| = | 25000 | − | X | × | 10 | X | − | 40 | × | 500 | + | 40 | × | 10 | X |
| = | 25000 | − | X | × | 10 | X | − | 20000 | + | 400 | X |
| = | 5000 | − | X | × | 10 | X | + | 400 | X |
| 5000 | − | X | × | 10 | X | + | 400 | X | = | 8000 |