| ( | 20 | − | 6 | x | ) | ( | 30 | − | 4 | x | ) | = | 4 | ÷ | 3 | × | 600 |
| 方程左边 = | 20 | ( | 30 | − | 4 | x | ) | − | 6 | x | ( | 30 | − | 4 | x | ) |
| = | 20 | × | 30 | − | 20 | × | 4 | x | − | 6 | x | ( | 30 | − | 4 | x | ) |
| = | 600 | − | 80 | x | − | 6 | x | ( | 30 | − | 4 | x | ) |
| = | 600 | − | 80 | x | − | 6 | x | × | 30 | + | 6 | x | × | 4 | x |
| = | 600 | − | 80 | x | − | 180 | x | + | 24 | x | x |
| = | 600 | − | 260 | x | + | 24 | x | x |
| 600 | − | 260 | x | + | 24 | x | x | = | 4 | ÷ | 3 | × | 600 |
| 方程右边 = | 800 |
| 600 | − | 260 | x | + | 24 | x | x | = | 800 |