| ( | 100 | + | 20 | x | ) | ( | 12 | − | x | ) | = | 1400 |
| 方程左边 = | 100 | ( | 12 | − | x | ) | + | 20 | x | ( | 12 | − | x | ) |
| = | 100 | × | 12 | − | 100 | x | + | 20 | x | ( | 12 | − | x | ) |
| = | 1200 | − | 100 | x | + | 20 | x | ( | 12 | − | x | ) |
| = | 1200 | − | 100 | x | + | 20 | x | × | 12 | − | 20 | x | x |
| = | 1200 | − | 100 | x | + | 240 | x | − | 20 | x | x |
| = | 1200 | + | 140 | x | − | 20 | x | x |
| 1200 | + | 140 | x | − | 20 | x | x | = | 1400 |