| 9 | × | 9 | X | − | 5 | ( | X | + | 4 | ) | = | X | + | 4 |
| 方程左边 = | 81 | X | − | 5 | ( | X | + | 4 | ) |
| 81 | X | − | 5 | ( | X | + | 4 | ) | = | X | + | 4 |
| 方程左边 = | 81 | X | − | 5 | X | − | 5 | × | 4 |
| = | 81 | X | − | 5 | X | − | 20 |
| = | 76 | X | − | 20 |
| 76 | X | − | 20 | = | X | + | 4 |
| 76 | X | − | X | = | 4 | + | 20 |
| 75 | X | = | 4 | + | 20 |
| 75 | X | = | 24 |
| X | = | 24 | ÷ | 75 |
| = | 24 | × | 1 75 |
| = | 8 | × | 1 25 |
| X | = | 8 25 |
| X | = | 0.32 |