| ( | 100 | − | x | ) | ÷ | ( | 0 | − | x | ) | = | - | 4 |
| Multiply both sides of the equation by: | ( | 0 | − | x | ) |
| ( | 100 | − | x | ) | = | - | 4 | ( | 0 | − | x | ) |
| 100 | − | x | = | - | 4 | ( | 0 | − | x | ) |
| 100 | − | x | = | - | 4 | × | 0 | + | 4 | x |
| 100 | − | x | = | - | 0 | + | 4 | x |
| - | x | − | 4 | x | = | - | 100 |
| - | 5 | x | = | - | 100 |
| 100 | = | 5 | x |
| 5 | x | = | 100 |
| x | = | 100 | ÷ | 5 |
| = | 100 | × | 1 5 |
| = | 20 | × | 1 |
| x | = | 20 |