| ( | 240 | − | 20 | x | ) | ( | 150 | − | 25 | x | ) | = | 0 |
| 方程左边 = | 240 | ( | 150 | − | 25 | x | ) | − | 20 | x | ( | 150 | − | 25 | x | ) |
| = | 240 | × | 150 | − | 240 | × | 25 | x | − | 20 | x | ( | 150 | − | 25 | x | ) |
| = | 36000 | − | 6000 | x | − | 20 | x | ( | 150 | − | 25 | x | ) |
| = | 36000 | − | 6000 | x | − | 20 | x | × | 150 | + | 20 | x | × | 25 | x |
| = | 36000 | − | 6000 | x | − | 3000 | x | + | 500 | x | x |
| = | 36000 | − | 9000 | x | + | 500 | x | x |
| 36000 | − | 9000 | x | + | 500 | x | x | = | 0 |