| 600 | ( | 30 | − | x | ) | = | 400 | ( | 30 | − | x | ) |
| Left side of the equation = | 600 | × | 30 | − | 600 | x |
| = | 18000 | − | 600 | x |
| 18000 | − | 600 | x | = | 400 | ( | 30 | − | x | ) |
| Right side of the equation = | 400 | × | 30 | − | 400 | x |
| = | 12000 | − | 400 | x |
| 18000 | − | 600 | x | = | 12000 | − | 400 | x |
| - | 600 | x | + | 400 | x | = | 12000 | − | 18000 |
| - | 200 | x | = | 12000 | − | 18000 |
| - | 200 | x | = | - | 6000 |
| 6000 | = | 200 | x |
| 200 | x | = | 6000 |
| x | = | 6000 | ÷ | 200 |
| = | 6000 | × | 1 200 |
| = | 30 | × | 1 |
| x | = | 30 |