| 6 | × | 5 | X | − | 4 | ( | X | + | 5 | ) | = | 21 | X |
| Left side of the equation = | 30 | X | − | 4 | ( | X | + | 5 | ) |
| 30 | X | − | 4 | ( | X | + | 5 | ) | = | 21 | X |
| Left side of the equation = | 30 | X | − | 4 | X | − | 4 | × | 5 |
| = | 30 | X | − | 4 | X | − | 20 |
| = | 26 | X | − | 20 |
| 26 | X | − | 20 | = | 21 | X |
| 26 | X | − | 21 | X | = | 20 |
| 5 | X | = | 20 |
| X | = | 20 | ÷ | 5 |
| = | 20 | × | 1 5 |
| = | 4 | × | 1 |
| X | = | 4 |