| 450 | x | ÷ | 3 | = | 300 | ( | 21 | − | x | ) | ÷ | 5 |
| Left side of the equation = | 150 | x |
| 150 | x | = | 300 | ( | 21 | − | x | ) | ÷ | 5 |
| Right side of the equation = | 60 | ( | 21 | − | x | ) |
| 150 | x | = | 60 | ( | 21 | − | x | ) |
| Right side of the equation = | 60 | × | 21 | − | 60 | x |
| = | 1260 | − | 60 | x |
| 150 | x | = | 1260 | − | 60 | x |
| 150 | x | + | 60 | x | = | 1260 |
| 210 | x | = | 1260 |
| x | = | 1260 | ÷ | 210 |
| = | 1260 | × | 1 210 |
| = | 6 | × | 1 |
| x | = | 6 |