| 75 | − | x | = | 180 | − | ( | 180 | − | x | ) | ÷ | 2 | − | 75 |
| 方程右边 = | 105 | − | ( | 180 | − | x | ) | × | 1 2 |
| 75 | − | x | = | 105 | − | ( | 180 | − | x | ) | × | 1 2 |
| 方程右边 = | 105 | − | 180 | × | 1 2 | + | x | × | 1 2 |
| = | 105 | − | 90 | + | x | × | 1 2 |
| = | 15 | + | 1 2 | x |
| 75 | − | x | = | 15 | + | 1 2 | x |
| - | x | − | 1 2 | x | = | 15 | − | 75 |
| - | 3 2 | x | = | 15 | − | 75 |
| - | 3 2 | x | = | - | 60 |
| 60 | = | 3 2 | x |
3 2 | x | = | 60 |
| x | = | 60 | ÷ | 3 2 |
| = | 60 | × | 2 3 |
| = | 20 | × | 2 |
| x | = | 40 |