| ( | 60 | + | X | − | 40 | ) | ( | 300 | − | 10 | X | ) | = | 0 |
| 方程左边 = | 60 | ( | 300 | − | 10 | X | ) | + | X | ( | 300 | − | 10 | X | ) | − | 40 | ( | 300 | − | 10 | X | ) |
| = | 60 | × | 300 | − | 60 | × | 10 | X | + | X | ( | 300 | − | 10 | X | ) | − | 40 | ( | 300 | − | 10 | X | ) |
| = | 18000 | − | 600 | X | + | X | ( | 300 | − | 10 | X | ) | − | 40 | ( | 300 | − | 10 | X | ) |
| = | 18000 | − | 600 | X | + | X | × | 300 | − | X | × | 10 | X | − | 40 | ( | 300 | − | 10 | X | ) |
| = | 18000 | − | 300 | X | − | X | × | 10 | X | − | 40 | ( | 300 | − | 10 | X | ) |
| = | 18000 | − | 300 | X | − | X | × | 10 | X | − | 40 | × | 300 | + | 40 | × | 10 | X |
| = | 18000 | − | 300 | X | − | X | × | 10 | X | − | 12000 | + | 400 | X |
| = | 6000 | + | 100 | X | − | X | × | 10 | X |
| 6000 | + | 100 | X | − | X | × | 10 | X | = | 0 |