| 12 | × | 7 10 | x | − | 12 | ( | x | − | 40 | ) | = | 6 | ( | x | − | 20 | ) | − | 6 | ( | x | − | 40 | ) |
| 方程左边 = | 42 5 | x | − | 12 | ( | x | − | 40 | ) |
42 5 | x | − | 12 | ( | x | − | 40 | ) | = | 6 | ( | x | − | 20 | ) | − | 6 | ( | x | − | 40 | ) |
| 方程左边 = | 42 5 | x | − | 12 | x | + | 12 | × | 40 |
| = | 42 5 | x | − | 12 | x | + | 480 |
| = | - | 18 5 | x | + | 480 |
| - | 18 5 | x | + | 480 | = | 6 | ( | x | − | 20 | ) | − | 6 | ( | x | − | 40 | ) |
| 方程右边 = | 6 | x | − | 6 | × | 20 | − | 6 | ( | x | − | 40 | ) |
| = | 6 | x | − | 120 | − | 6 | ( | x | − | 40 | ) |
| = | 6 | x | − | 120 | − | 6 | x | + | 6 | × | 40 |
| = | 6 | x | − | 120 | − | 6 | x | + | 240 |
| = | 0 | x | + | 120 |
| - | 18 5 | x | + | 480 | = | 0 | x | + | 120 |
| - | 18 5 | x | = | 120 | − | 480 |
| - | 18 5 | x | = | - | 360 |
| 360 | = | 18 5 | x |
18 5 | x | = | 360 |
| x | = | 360 | ÷ | 18 5 |
| = | 360 | × | 5 18 |
| = | 20 | × | 5 |
| x | = | 100 |