| ( | 40 | − | 2 | x | ) | ( | 60 | − | 2 | x | ) | = | 25 | × | 60 |
| 方程左边 = | 40 | ( | 60 | − | 2 | x | ) | − | 2 | x | ( | 60 | − | 2 | x | ) |
| = | 40 | × | 60 | − | 40 | × | 2 | x | − | 2 | x | ( | 60 | − | 2 | x | ) |
| = | 2400 | − | 80 | x | − | 2 | x | ( | 60 | − | 2 | x | ) |
| = | 2400 | − | 80 | x | − | 2 | x | × | 60 | + | 2 | x | × | 2 | x |
| = | 2400 | − | 80 | x | − | 120 | x | + | 4 | x | x |
| = | 2400 | − | 200 | x | + | 4 | x | x |
| 2400 | − | 200 | x | + | 4 | x | x | = | 25 | × | 60 |
| 方程右边 = | 1500 |
| 2400 | − | 200 | x | + | 4 | x | x | = | 1500 |