| ( | 20 | x | + | 20 | ) | ( | 40 | − | 5 | x | ) | = | 200 |
| 方程左边 = | 20 | x | ( | 40 | − | 5 | x | ) | + | 20 | ( | 40 | − | 5 | x | ) |
| = | 20 | x | × | 40 | − | 20 | x | × | 5 | x | + | 20 | ( | 40 | − | 5 | x | ) |
| = | 800 | x | − | 100 | x | x | + | 20 | ( | 40 | − | 5 | x | ) |
| = | 800 | x | − | 100 | x | x | + | 20 | × | 40 | − | 20 | × | 5 | x |
| = | 800 | x | − | 100 | x | x | + | 800 | − | 100 | x |
| = | 700 | x | − | 100 | x | x | + | 800 |
| 700 | x | − | 100 | x | x | + | 800 | = | 200 |