| 20 | ( | 20 | − | x | ) | + | 10 | ( | x | ÷ | 5 | ) | ( | 20 | − | x | ) | = | 400 |
| 方程左边 = | 20 | × | 20 | − | 20 | x | + | 10 | ( | x | ÷ | 5 | ) | ( | 20 | − | x | ) |
| = | 400 | − | 20 | x | + | 10 | ( | x | ÷ | 5 | ) | ( | 20 | − | x | ) |
| = | 400 | − | 20 | x | + | 10 | x | ÷ | 5 | × | ( | 20 | − | x | ) |
| = | 400 | − | 20 | x | + | 2 | x | ( | 20 | − | x | ) |
| = | 400 | − | 20 | x | + | 2 | x | × | 20 | − | 2 | x | x |
| = | 400 | − | 20 | x | + | 40 | x | − | 2 | x | x |
| = | 400 | + | 20 | x | − | 2 | x | x |
| 400 | + | 20 | x | − | 2 | x | x | = | 400 |