| ( | x | − | 5 | ) | ( | 20 | ( | 30 | − | x | ) | + | 300 | ) | = | 7820 |
| 方程左边 = | x | ( | 20 | ( | 30 | − | x | ) | + | 300 | ) | − | 5 | ( | 20 | ( | 30 | − | x | ) | + | 300 | ) |
| = | x | × | 20 | ( | 30 | − | x | ) | + | x | × | 300 | − | 5 | ( | 20 | ( | 30 | − | x | ) | + | 300 | ) |
| = | x | × | 20 | × | 30 | − | x | × | 20 | x | + | 300 | x | − | 5 | ( | 20 | ( | 30 | − | x | ) | + | 300 | ) |
| = | x | × | 600 | − | x | × | 20 | x | + | 300 | x | − | 5 | ( | 20 | ( | 30 | − | x | ) | + | 300 | ) |
| = | 900 | x | − | x | × | 20 | x | − | 5 | ( | 20 | ( | 30 | − | x | ) | + | 300 | ) |
| = | 900 | x | − | x | × | 20 | x | − | 5 | × | 20 | ( | 30 | − | x | ) | − | 5 | × | 300 |
| = | 900 | x | − | x | × | 20 | x | − | 100 | ( | 30 | − | x | ) | − | 1500 |
| = | 900 | x | − | x | × | 20 | x | − | 100 | × | 30 | + | 100 | x | − | 1500 |
| = | 900 | x | − | x | × | 20 | x | − | 3000 | + | 100 | x | − | 1500 |
| = | 1000 | x | − | x | × | 20 | x | − | 4500 |
| 1000 | x | − | x | × | 20 | x | − | 4500 | = | 7820 |