| ( | 25 | + | x | ) | ÷ | ( | 45 | + | x | ) | = | 7 | ÷ | 12 |
| 方程两边同时乘以: | ( | 45 | + | x | ) |
| ( | 25 | + | x | ) | = | 7 | ÷ | 12 | × | ( | 45 | + | x | ) |
| 25 | + | x | = | 7 | ÷ | 12 | × | ( | 45 | + | x | ) |
| 25 | + | x | = | 7 | ÷ | 12 | × | 45 | + | 7 | ÷ | 12 | × | x |
| 25 | + | x | = | 105 4 | + | 7 12 | x |
| x | − | 7 12 | x | = | 105 4 | − | 25 |
5 12 | x | = | 105 4 | − | 25 |
5 12 | x | = | 5 4 |
| x | = | 5 4 | ÷ | 5 12 |
| = | 5 4 | × | 12 5 |
| = | 1 | × | 3 |
| x | = | 3 |