| ( | 40 | + | x | ) | ÷ | ( | 13 | + | x | ) | = | 4 |
| Multiply both sides of the equation by: | ( | 13 | + | x | ) |
| ( | 40 | + | x | ) | = | 4 | ( | 13 | + | x | ) |
| 40 | + | x | = | 4 | ( | 13 | + | x | ) |
| 40 | + | x | = | 4 | × | 13 | + | 4 | x |
| 40 | + | x | = | 52 | + | 4 | x |
| x | − | 4 | x | = | 52 | − | 40 |
| - | 3 | x | = | 52 | − | 40 |
| - | 3 | x | = | 12 |
| - | 12 | = | 3 | x |
| 3 | x | = | - | 12 |
| x | = | - | 12 | ÷ | 3 |
| = | - | 12 | × | 1 3 |
| = | - | 4 | × | 1 |
| x | = | - | 4 |