| ( | 1 | − | 20 100 | ) | ( | 1 | + | x% | ) | = | 1 |
| 方程左边 = | 1 | ( | 1 | + | x% | ) | − | 20 100 | ( | 1 | + | x% | ) |
| = | 1 | × | 1 | + | 1 | × | x% | − | 20 100 | ( | 1 | + | x% | ) |
| = | 1 | + | 1 100 | x | − | 20 100 | ( | 1 | + | x% | ) |
| = | 1 | + | 1 100 | x | − | 20 100 | × | 1 | − | 20 100 | × | x% |
| = | 1 | + | 1 100 | x | − | 1 5 | − | 1 500 | x |
| = | 4 5 | + | 1 125 | x |
4 5 | + | 1 125 | x | = | 1 |
1 125 | x | = | 1 | − | 4 5 |
1 125 | x | = | 1 5 |
| x | = | 1 5 | ÷ | 1 125 |
| = | 1 5 | × | 125 |
| = | 1 | × | 25 |
| x | = | 25 |