| ( | 240 | + | 20 | x | ) | ( | 20 | − | x | ) | = | 6500 |
| 方程左边 = | 240 | ( | 20 | − | x | ) | + | 20 | x | ( | 20 | − | x | ) |
| = | 240 | × | 20 | − | 240 | x | + | 20 | x | ( | 20 | − | x | ) |
| = | 4800 | − | 240 | x | + | 20 | x | ( | 20 | − | x | ) |
| = | 4800 | − | 240 | x | + | 20 | x | × | 20 | − | 20 | x | x |
| = | 4800 | − | 240 | x | + | 400 | x | − | 20 | x | x |
| = | 4800 | + | 160 | x | − | 20 | x | x |
| 4800 | + | 160 | x | − | 20 | x | x | = | 6500 |