| ( | X | + | 40 | − | 30 | ) | ( | 300 | − | 10 | X | ) | = | 3360 |
| 方程左边 = | X | ( | 300 | − | 10 | X | ) | + | 40 | ( | 300 | − | 10 | X | ) | − | 30 | ( | 300 | − | 10 | X | ) |
| = | X | × | 300 | − | X | × | 10 | X | + | 40 | ( | 300 | − | 10 | X | ) | − | 30 | ( | 300 | − | 10 | X | ) |
| = | 300 | X | − | X | × | 10 | X | + | 40 | × | 300 | − | 40 | × | 10 | X | − | 30 | ( | 300 | − | 10 | X | ) |
| = | 300 | X | − | X | × | 10 | X | + | 12000 | − | 400 | X | − | 30 | ( | 300 | − | 10 | X | ) |
| = | - | 100 | X | − | X | × | 10 | X | + | 12000 | − | 30 | ( | 300 | − | 10 | X | ) |
| = | - | 100 | X | − | X | × | 10 | X | + | 12000 | − | 30 | × | 300 | + | 30 | × | 10 | X |
| = | - | 100 | X | − | X | × | 10 | X | + | 12000 | − | 9000 | + | 300 | X |
| = | 200 | X | − | X | × | 10 | X | + | 3000 |
| 200 | X | − | X | × | 10 | X | + | 3000 | = | 3360 |