| X | = | ( | 21 | − | 1 | ) | ÷ | ( | 3 | + | 1 | ) | × | 3 | + | 1 |
| Multiply both sides of the equation by: | ( | 3 | + | 1 | ) |
| X | ( | 3 | + | 1 | ) | = | ( | 21 | − | 1 | ) | × | 3 | + | 1 | ( | 3 | + | 1 | ) |
| X | × | 3 | + | X | × | 1 | = | ( | 21 | − | 1 | ) | × | 3 | + | 1 | ( | 3 | + | 1 | ) |
| X | × | 3 | + | X | × | 1 | = | 21 | × | 3 | − | 1 | × | 3 | + | 1 | ( | 3 | + | 1 | ) |
| X | × | 3 | + | X | × | 1 | = | 63 | − | 3 | + | 1 | ( | 3 | + | 1 | ) |
| 4 | X | = | 60 | + | 1 | ( | 3 | + | 1 | ) |
| 4 | X | = | 60 | + | 1 | × | 3 | + | 1 | × | 1 |
| 4 | X | = | 60 | + | 3 | + | 1 |
| 4 | X | = | 64 |
| X | = | 64 | ÷ | 4 |
| = | 64 | × | 1 4 |
| = | 16 | × | 1 |
| X | = | 16 |