| ( | 110 | − | x | ) | × | 90 | = | ( | 90 | − | x | ) | × | 210 |
| 方程左边 = | 110 | × | 90 | − | x | × | 90 |
| = | 9900 | − | x | × | 90 |
| 9900 | − | 90 | x | = | ( | 90 | − | x | ) | × | 210 |
| 方程右边 = | 90 | × | 210 | − | x | × | 210 |
| = | 18900 | − | x | × | 210 |
| 9900 | − | 90 | x | = | 18900 | − | 210 | x |
| - | 90 | x | + | 210 | x | = | 18900 | − | 9900 |
| 120 | x | = | 18900 | − | 9900 |
| 120 | x | = | 9000 |
| x | = | 9000 | ÷ | 120 |
| = | 9000 | × | 1 120 |
| = | 75 | × | 1 |
| x | = | 75 |