| 30 | x | + | 55 | ( | 1200 | − | x | ) | − | ( | 20 | x | + | 40 | ( | 1200 | − | x | ) | ) | = | 9 20 | ( | 20 | x | + | 40 | ( | 1200 | − | x | ) | ) |
| Left side of the equation = | 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 | ( | 20 | x | + | 40 | ( | 1200 | − | x | ) | ) |
| Right side of the equation = | 9 20 | × | 20 | x | + | 9 20 | × | 40 | ( | 1200 | − | x | ) |
| = | 9 | x | + | 18 | ( | 1200 | − | x | ) |
| = | 9 | x | + | 18 | × | 1200 | − | 18 | x |
| = | 9 | x | + | 21600 | − | 18 | x |
| = | - | 9 | x | + | 21600 |
| - | 5 | x | + | 18000 | = | - | 9 | x | + | 21600 |
| - | 5 | x | + | 9 | x | = | 21600 | − | 18000 |
| 4 | x | = | 21600 | − | 18000 |
| 4 | x | = | 3600 |
| x | = | 3600 | ÷ | 4 |
| = | 3600 | × | 1 4 |
| = | 900 | × | 1 |
| x | = | 900 |