| ( | 300 | − | 10 | x | ) | ( | 10 | − | x | ) | = | 3750 |
| 方程左边 = | 300 | ( | 10 | − | x | ) | − | 10 | x | ( | 10 | − | x | ) |
| = | 300 | × | 10 | − | 300 | x | − | 10 | x | ( | 10 | − | x | ) |
| = | 3000 | − | 300 | x | − | 10 | x | ( | 10 | − | x | ) |
| = | 3000 | − | 300 | x | − | 10 | x | × | 10 | + | 10 | x | x |
| = | 3000 | − | 300 | x | − | 100 | x | + | 10 | x | x |
| = | 3000 | − | 400 | x | + | 10 | x | x |
| 3000 | − | 400 | x | + | 10 | x | x | = | 3750 |