| ( | 100 | + | 10 | x | ) | ( | 40 | − | x | ) | = | 4320 |
| 方程左边 = | 100 | ( | 40 | − | x | ) | + | 10 | x | ( | 40 | − | x | ) |
| = | 100 | × | 40 | − | 100 | x | + | 10 | x | ( | 40 | − | x | ) |
| = | 4000 | − | 100 | x | + | 10 | x | ( | 40 | − | x | ) |
| = | 4000 | − | 100 | x | + | 10 | x | × | 40 | − | 10 | x | x |
| = | 4000 | − | 100 | x | + | 400 | x | − | 10 | x | x |
| = | 4000 | + | 300 | x | − | 10 | x | x |
| 4000 | + | 300 | x | − | 10 | x | x | = | 4320 |