| ( | 12 | + | x | ) | ÷ | ( | 20 | + | x | ) | = | 4 5 |
| Multiply both sides of the equation by: | ( | 20 | + | x | ) |
| ( | 12 | + | x | ) | = | 4 5 | ( | 20 | + | x | ) |
| 12 | + | x | = | 4 5 | ( | 20 | + | x | ) |
| 12 | + | x | = | 4 5 | × | 20 | + | 4 5 | x |
| 12 | + | x | = | 16 | + | 4 5 | x |
| x | − | 4 5 | x | = | 16 | − | 12 |
1 5 | x | = | 16 | − | 12 |
1 5 | x | = | 4 |
| x | = | 4 | ÷ | 1 5 |
| = | 4 | × | 5 |
| x | = | 20 |