| ( | 3 4 | x | − | 1200 | ) | × | 4 | = | ( | 3 4 | x | − | 1500 | ) | × | 3 |
| 方程左边 = | 3 4 | x | × | 4 | − | 1200 | × | 4 |
| = | 3 | x | − | 4800 |
| 3 | x | − | 4800 | = | ( | 3 4 | x | − | 1500 | ) | × | 3 |
| 方程右边 = | 3 4 | x | × | 3 | − | 1500 | × | 3 |
| = | 9 4 | x | − | 4500 |
| 3 | x | − | 4800 | = | 9 4 | x | − | 4500 |
| 3 | x | − | 9 4 | x | = | - | 4500 | + | 4800 |
3 4 | x | = | - | 4500 | + | 4800 |
3 4 | x | = | 300 |
| x | = | 300 | ÷ | 3 4 |
| = | 300 | × | 4 3 |
| = | 100 | × | 4 |
| x | = | 400 |