| ( | x | − | 30 | ) | ( | 300 | − | ( | x | − | 40 | ) | × | 10 | ) | = | 3360 |
| 方程左边 = | x | ( | 300 | − | ( | x | − | 40 | ) | × | 10 | ) | − | 30 | ( | 300 | − | ( | x | − | 40 | ) | × | 10 | ) |
| = | x | × | 300 | − | x | ( | x | − | 40 | ) | × | 10 | − | 30 | ( | 300 | − | ( | x | − | 40 | ) | × | 10 | ) |
| = | 300 | x | − | x | x | × | 10 | + | x | × | 40 | × | 10 | − | 30 | ( | 300 | − | ( | x | − | 40 | ) | × | 10 | ) |
| = | 300 | x | − | x | x | × | 10 | + | x | × | 400 | − | 30 | ( | 300 | − | ( | x | − | 40 | ) | × | 10 | ) |
| = | 700 | x | − | x | x | × | 10 | − | 30 | ( | 300 | − | ( | x | − | 40 | ) | × | 10 | ) |
| = | 700 | x | − | x | x | × | 10 | − | 30 | × | 300 | + | 30 | ( | x | − | 40 | ) | × | 10 |
| = | 700 | x | − | x | x | × | 10 | − | 9000 | + | 300 | ( | x | − | 40 | ) |
| = | 700 | x | − | x | x | × | 10 | − | 9000 | + | 300 | x | − | 300 | × | 40 |
| = | 700 | x | − | x | x | × | 10 | − | 9000 | + | 300 | x | − | 12000 |
| = | 1000 | x | − | x | x | × | 10 | − | 21000 |
| 1000 | x | − | x | x | × | 10 | − | 21000 | = | 3360 |