| 30 | x | + | 55 | ( | 1200 | − | x | ) | − | ( | 20 | x | + | 40 | ( | 1200 | − | x | ) | ) | = | 9 20 | ( | 30 | x | + | 55 | ( | 1200 | − | x | ) | ) |
| 方程左边 = | 30 | x | + | 55 | × | 1200 | − | 55 | x | − | ( | 20 | x | + | 40 | ( | 1200 | − | x | ) | ) |
| = | 30 | x | + | 66000 | − | 55 | x | − | ( | 20 | x | + | 40 | ( | 1200 | − | x | ) | ) |
| = | - | 25 | x | + | 66000 | − | ( | 20 | x | + | 40 | ( | 1200 | − | x | ) | ) |
| = | - | 25 | x | + | 66000 | − | 20 | x | − | 40 | ( | 1200 | − | x | ) |
| = | - | 45 | x | + | 66000 | − | 40 | ( | 1200 | − | x | ) |
| = | - | 45 | x | + | 66000 | − | 40 | × | 1200 | + | 40 | x |
| = | - | 45 | x | + | 66000 | − | 48000 | + | 40 | x |
| = | - | 5 | x | + | 18000 |
| - | 5 | x | + | 18000 | = | 9 20 | ( | 30 | x | + | 55 | ( | 1200 | − | x | ) | ) |
| 方程右边 = | 9 20 | × | 30 | x | + | 9 20 | × | 55 | ( | 1200 | − | x | ) |
| = | 27 2 | x | + | 99 4 | ( | 1200 | − | x | ) |
| = | 27 2 | x | + | 99 4 | × | 1200 | − | 99 4 | x |
| = | 27 2 | x | + | 29700 | − | 99 4 | x |
| = | - | 45 4 | x | + | 29700 |
| - | 5 | x | + | 18000 | = | - | 45 4 | x | + | 29700 |
| - | 5 | x | + | 45 4 | x | = | 29700 | − | 18000 |
25 4 | x | = | 29700 | − | 18000 |
25 4 | x | = | 11700 |
| x | = | 11700 | ÷ | 25 4 |
| = | 11700 | × | 4 25 |
| = | 468 | × | 4 |
| x | = | 1872 |