| 1500 | x | + | 2100 | ( | 50 | − | x | ) | = | 90000 |
| Left side of the equation = | 1500 | x | + | 2100 | × | 50 | − | 2100 | x |
| = | 1500 | x | + | 105000 | − | 2100 | x |
| = | - | 600 | x | + | 105000 |
| - | 600 | x | + | 105000 | = | 90000 |
| - | 600 | x | = | 90000 | − | 105000 |
| - | 600 | x | = | - | 15000 |
| 15000 | = | 600 | x |
| 600 | x | = | 15000 |
| x | = | 15000 | ÷ | 600 |
| = | 15000 | × | 1 600 |
| = | 25 | × | 1 |
| x | = | 25 |