| 20 | × | 200 | ( | 1 | − | 20 100 | ) | + | 10 | × | 400 | ( | 1 | − | a% | ) | = | 6000 |
| 方程左边 = | 4000 | ( | 1 | − | 20 100 | ) | + | 4000 | ( | 1 | − | a% | ) |
| 4000 | ( | 1 | − | 20 100 | ) | + | 4000 | ( | 1 | − | a% | ) | = | 6000 |
| 方程左边 = | 4000 | × | 1 | − | 4000 | × | 20 100 | + | 4000 | ( | 1 | − | a% | ) |
| = | 4000 | − | 800 | + | 4000 | ( | 1 | − | a% | ) |
| = | 3200 | + | 4000 | ( | 1 | − | a% | ) |
| = | 3200 | + | 4000 | × | 1 | − | 4000 | × | a% |
| = | 3200 | + | 4000 | − | 40 | a |
| = | 7200 | − | 40 | a |
| 7200 | − | 40 | a | = | 6000 |
| - | 40 | a | = | 6000 | − | 7200 |
| - | 40 | a | = | - | 1200 |
| 1200 | = | 40 | a |
| 40 | a | = | 1200 |
| a | = | 1200 | ÷ | 40 |
| = | 1200 | × | 1 40 |
| = | 30 | × | 1 |
| a | = | 30 |