| 90 | = | ( | x | + | ( | 120 | − | x | ) | ÷ | 4 | ) | ÷ | 120 | × | 100 |
| 方程右边 = | ( | x | + | ( | 120 | − | x | ) | ÷ | 4 | ) | × | 5 6 |
| 90 | = | ( | x | + | ( | 120 | − | x | ) | ÷ | 4 | ) | × | 5 6 |
| 方程右边 = | x | × | 5 6 | + | ( | 120 | − | x | ) | ÷ | 4 | × | 5 6 |
| = | x | × | 5 6 | + | ( | 120 | − | x | ) | × | 5 24 |
| = | 5 6 | x | + | 120 | × | 5 24 | − | x | × | 5 24 |
| = | 5 6 | x | + | 25 | − | x | × | 5 24 |
| = | 5 8 | x | + | 25 |
| 90 | = | 5 8 | x | + | 25 |
| - | 5 8 | x | = | 25 | − | 90 |
| - | 5 8 | x | = | - | 65 |
| 65 | = | 5 8 | x |
5 8 | x | = | 65 |
| x | = | 65 | ÷ | 5 8 |
| = | 65 | × | 8 5 |
| = | 13 | × | 8 |
| x | = | 104 |