| 7 | ( | 90 | − | x | ) | = | 3 | ( | 180 | − | x | ) |
| 方程左边 = | 7 | × | 90 | − | 7 | x |
| = | 630 | − | 7 | x |
| 630 | − | 7 | x | = | 3 | ( | 180 | − | x | ) |
| 方程右边 = | 3 | × | 180 | − | 3 | x |
| = | 540 | − | 3 | x |
| 630 | − | 7 | x | = | 540 | − | 3 | x |
| - | 7 | x | + | 3 | x | = | 540 | − | 630 |
| - | 4 | x | = | 540 | − | 630 |
| - | 4 | x | = | - | 90 |
| 90 | = | 4 | x |
| 4 | x | = | 90 |
| x | = | 90 | ÷ | 4 |
| = | 90 | × | 1 4 |
| = | 45 | × | 1 2 |
| x | = | 45 2 |
| x | = | 22.5 |