| ( | 40 | + | 10 | X | ) | ÷ | ( | 13 | + | X | ) | = | 4 |
| Multiply both sides of the equation by: | ( | 13 | + | X | ) |
| ( | 40 | + | 10 | X | ) | = | 4 | ( | 13 | + | X | ) |
| 40 | + | 10 | X | = | 4 | ( | 13 | + | X | ) |
| 40 | + | 10 | X | = | 4 | × | 13 | + | 4 | X |
| 40 | + | 10 | X | = | 52 | + | 4 | X |
| 10 | X | − | 4 | X | = | 52 | − | 40 |
| 6 | X | = | 52 | − | 40 |
| 6 | X | = | 12 |
| X | = | 12 | ÷ | 6 |
| = | 12 | × | 1 6 |
| = | 2 | × | 1 |
| X | = | 2 |