| ( | 3 5 | − | x | ) | ( | 500 | + | 100 | x | ) | = | 240 |
| 方程左边 = | 3 5 | ( | 500 | + | 100 | x | ) | − | x | ( | 500 | + | 100 | x | ) |
| = | 3 5 | × | 500 | + | 3 5 | × | 100 | x | − | x | ( | 500 | + | 100 | x | ) |
| = | 300 | + | 60 | x | − | x | ( | 500 | + | 100 | x | ) |
| = | 300 | + | 60 | x | − | x | × | 500 | − | x | × | 100 | x |
| = | 300 | − | 440 | x | − | x | × | 100 | x |
| 300 | − | 440 | x | − | x | × | 100 | x | = | 240 |