| 300 | × | 1 | a | = | 250 | × | 3 2 | ( | a | − | 20 | ) |
| 方程左边 = | 300 | a |
| 300 | a | = | 250 | × | 3 2 | ( | a | − | 20 | ) |
| 方程右边 = | 375 | ( | a | − | 20 | ) |
| 300 | a | = | 375 | ( | a | − | 20 | ) |
| 方程右边 = | 375 | a | − | 375 | × | 20 |
| = | 375 | a | − | 7500 |
| 300 | a | = | 375 | a | − | 7500 |
| 300 | a | − | 375 | a | = | - | 7500 |
| - | 75 | a | = | - | 7500 |
| 7500 | = | 75 | a |
| 75 | a | = | 7500 |
| a | = | 7500 | ÷ | 75 |
| = | 7500 | × | 1 75 |
| = | 100 | × | 1 |
| a | = | 100 |