| ( | 7890 | − | 88 | X | − | 90 | ) | ÷ | ( | 90 | − | X | ) | = | 88 |
| 方程两边同时乘以: | ( | 90 | − | X | ) |
| ( | 7890 | − | 88 | X | − | 90 | ) | = | 88 | ( | 90 | − | X | ) |
| 7890 | − | 88 | X | − | 90 | = | 88 | ( | 90 | − | X | ) |
| 7890 | − | 88 | X | − | 90 | = | 88 | × | 90 | − | 88 | X |
| 7890 | − | 88 | X | − | 90 | = | 7920 | − | 88 | X |
| 7800 | − | 88 | X | = | 7920 | − | 88 | X |
| - | 88 | X | + | 88 | X | = | 7920 | − | 7800 |
| - | 0 | X | = | 7920 | − | 7800 |
| - | 0 | X | = | 120 |