| ( | 120 | + | x | ) | = | 7 20 | ( | 300 | + | 10 | x | ) |
| 方程左边 = | 120 | + | x |
| 120 | + | x | = | 7 20 | ( | 300 | + | 10 | x | ) |
| 方程右边 = | 7 20 | × | 300 | + | 7 20 | × | 10 | x |
| = | 105 | + | 7 2 | x |
| 120 | + | x | = | 105 | + | 7 2 | x |
| x | − | 7 2 | x | = | 105 | − | 120 |
| - | 5 2 | x | = | 105 | − | 120 |
| - | 5 2 | x | = | - | 15 |
| 15 | = | 5 2 | x |
5 2 | x | = | 15 |
| x | = | 15 | ÷ | 5 2 |
| = | 15 | × | 2 5 |
| = | 3 | × | 2 |
| x | = | 6 |