| ( | 128 | − | X | ) | ( | 1 | − | 20 100 | ) | = | X | − | 20 |
| 方程左边 = | 128 | ( | 1 | − | 20 100 | ) | − | X | ( | 1 | − | 20 100 | ) |
| = | 128 | × | 1 | − | 128 | × | 20 100 | − | X | ( | 1 | − | 20 100 | ) |
| = | 128 | − | 128 5 | − | X | ( | 1 | − | 20 100 | ) |
| = | 512 5 | − | X | ( | 1 | − | 20 100 | ) |
| = | 512 5 | − | X | × | 1 | + | X | × | 20 100 |
| = | 512 5 | − | 4 5 | X |
512 5 | − | 4 5 | X | = | X | − | 20 |
| - | 4 5 | X | − | X | = | - | 20 | − | 512 5 |
| - | 9 5 | X | = | - | 20 | − | 512 5 |
| - | 9 5 | X | = | - | 612 5 |
612 5 | = | 9 5 | X |
9 5 | X | = | 612 5 |
| X | = | 612 5 | ÷ | 9 5 |
| = | 612 5 | × | 5 9 |
| = | 68 | × | 1 |
| X | = | 68 |
512 5 | − | 4 5 | X | = | X | − | 20 |
| - | 4 5 | X | − | X | = | - | 20 | − | 512 5 |
| - | 9 5 | X | = | - | 20 | − | 512 5 |
| - | 9 5 | X | = | - | 612 5 |
612 5 | = | 9 5 | X |
9 5 | X | = | 612 5 |
| X | = | 612 5 | ÷ | 9 5 |
| = | 612 5 | × | 5 9 |
| = | 68 | × | 1 |
| X | = | 68 |
9 5 | X | = | 612 5 |
| X | = | 612 5 | ÷ | 9 5 |
| = | 612 5 | × | 5 9 |
| = | 68 | × | 1 |
| X | = | 68 |
| X | = | 68 |