| x | − | ( | 80 | − | x | ) | = | ( | 80 | − | x | ) | − | ( | ( | 80 | − | x | ) | ÷ | 2 | ) |
| 方程左边 = | x | − | 80 | + | x |
| = | 2 | x | − | 80 |
| 2 | x | − | 80 | = | ( | 80 | − | x | ) | − | ( | ( | 80 | − | x | ) | ÷ | 2 | ) |
| 方程右边 = | 80 | − | x | − | ( | ( | 80 | − | x | ) | ÷ | 2 | ) |
| = | 80 | − | x | − | ( | 80 | − | x | ) | ÷ | 2 |
| = | 80 | − | x | − | 80 | × | 1 2 | + | x | × | 1 2 |
| = | 80 | − | x | − | 40 | + | x | × | 1 2 |
| = | 40 | − | 1 2 | x |
| 2 | x | − | 80 | = | 40 | − | 1 2 | x |
| 2 | x | + | 1 2 | x | = | 40 | + | 80 |
5 2 | x | = | 40 | + | 80 |
5 2 | x | = | 120 |
| x | = | 120 | ÷ | 5 2 |
| = | 120 | × | 2 5 |
| = | 24 | × | 2 |
| x | = | 48 |