| 180 | ÷ | x | = | 210 | ÷ | ( | x | + | 20 | ) |
| Multiply both sides of the equation by: | x | , | ( | x | + | 20 | ) |
| 180 | ( | x | + | 20 | ) | = | 210 | x |
| 180 | x | + | 180 | × | 20 | = | 210 | x |
| 180 | x | + | 3600 | = | 210 | x |
| 180 | x | − | 210 | x | = | - | 3600 |
| - | 30 | x | = | - | 3600 |
| 3600 | = | 30 | x |
| 30 | x | = | 3600 |
| x | = | 3600 | ÷ | 30 |
| = | 3600 | × | 1 30 |
| = | 120 | × | 1 |
| x | = | 120 |