| ( | 1200 | + | X | ) | ÷ | ( | 3000 | + | X | ) | = | 3 | ÷ | 5 |
| 方程两边同时乘以: | ( | 3000 | + | X | ) |
| ( | 1200 | + | X | ) | = | 3 | ÷ | 5 | × | ( | 3000 | + | X | ) |
| 1200 | + | X | = | 3 | ÷ | 5 | × | ( | 3000 | + | X | ) |
| 1200 | + | X | = | 3 | ÷ | 5 | × | 3000 | + | 3 | ÷ | 5 | × | X |
| 1200 | + | X | = | 1800 | + | 3 5 | X |
| X | − | 3 5 | X | = | 1800 | − | 1200 |
2 5 | X | = | 1800 | − | 1200 |
2 5 | X | = | 600 |
| X | = | 600 | ÷ | 2 5 |
| = | 600 | × | 5 2 |
| = | 300 | × | 5 |
| X | = | 1500 |