| 3 | × | 450 | x | = | 5 | × | 300 | ( | 21 | − | x | ) |
| 方程左边 = | 1350 | x |
| 1350 | x | = | 5 | × | 300 | ( | 21 | − | x | ) |
| 方程右边 = | 1500 | ( | 21 | − | x | ) |
| 1350 | x | = | 1500 | ( | 21 | − | x | ) |
| 方程右边 = | 1500 | × | 21 | − | 1500 | x |
| = | 31500 | − | 1500 | x |
| 1350 | x | = | 31500 | − | 1500 | x |
| 1350 | x | + | 1500 | x | = | 31500 |
| 2850 | x | = | 31500 |
| x | = | 31500 | ÷ | 2850 |
| = | 31500 | × | 1 2850 |
| = | 210 | × | 1 19 |
| x | = | 210 19 |
| x | = | 11.052632 |