| - | 25 | x | + | 45 | ( | 100 | − | x | ) | = | 4200 |
| Left side of the equation = | - | 25 | x | + | 45 | × | 100 | − | 45 | x |
| = | - | 25 | x | + | 4500 | − | 45 | x |
| = | - | 70 | x | + | 4500 |
| - | 70 | x | + | 4500 | = | 4200 |
| - | 70 | x | = | 4200 | − | 4500 |
| - | 70 | x | = | - | 300 |
| 300 | = | 70 | x |
| 70 | x | = | 300 |
| x | = | 300 | ÷ | 70 |
| = | 300 | × | 1 70 |
| = | 30 | × | 1 7 |
| x | = | 30 7 |
| x | = | 4.285714 |