| ( | 10 | + | x | ) | ÷ | ( | 100 | + | x | ) | = | 1 5 |
| Multiply both sides of the equation by: | ( | 100 | + | x | ) |
| ( | 10 | + | x | ) | = | 1 5 | ( | 100 | + | x | ) |
| 10 | + | x | = | 1 5 | ( | 100 | + | x | ) |
| 10 | + | x | = | 1 5 | × | 100 | + | 1 5 | x |
| 10 | + | x | = | 20 | + | 1 5 | x |
| x | − | 1 5 | x | = | 20 | − | 10 |
4 5 | x | = | 20 | − | 10 |
4 5 | x | = | 10 |
| x | = | 10 | ÷ | 4 5 |
| = | 10 | × | 5 4 |
| = | 5 | × | 5 2 |
| x | = | 25 2 |
| x | = | 12.5 |