| ( | 40 | − | x | ÷ | 10 | ) | ( | 20 | + | x | ) | = | 1200 |
| 方程左边 = | 40 | ( | 20 | + | x | ) | − | x | ÷ | 10 | × | ( | 20 | + | x | ) |
| = | 40 | × | 20 | + | 40 | x | − | x | × | 1 10 | ( | 20 | + | x | ) |
| = | 800 | + | 40 | x | − | x | × | 1 10 | ( | 20 | + | x | ) |
| = | 800 | + | 40 | x | − | x | × | 1 10 | × | 20 | − | x | × | 1 10 | x |
| = | 800 | + | 40 | x | − | x | × | 2 | − | x | × | 1 10 | x |
| = | 800 | + | 38 | x | − | x | × | 1 10 | x |
| 800 | + | 38 | x | − | x | × | 1 10 | x | = | 1200 |