| 400 | × | 10 | ( | 1 | + | a% | ) | × | 12 5 | + | 500 | × | 10 | ( | 1 | + | a% | ) | = | 21600 | ( | 1 | + | 20 | ÷ | 9 | × | a% | ) |
| 方程左边 = | 9600 | ( | 1 | + | a% | ) | + | 5000 | ( | 1 | + | a% | ) |
| 9600 | ( | 1 | + | a% | ) | + | 5000 | ( | 1 | + | a% | ) | = | 21600 | ( | 1 | + | 20 | ÷ | 9 | × | a% | ) |
| 方程左边 = | 9600 | × | 1 | + | 9600 | × | a% | + | 5000 | ( | 1 | + | a% | ) |
| = | 9600 | + | 96 | a | + | 5000 | ( | 1 | + | a% | ) |
| = | 9600 | + | 96 | a | + | 5000 | × | 1 | + | 5000 | × | a% |
| = | 9600 | + | 96 | a | + | 5000 | + | 50 | a |
| = | 14600 | + | 146 | a |
| 14600 | + | 146 | a | = | 21600 | ( | 1 | + | 20 | ÷ | 9 | × | a% | ) |
| 方程右边 = | 21600 | × | 1 | + | 21600 | × | 20 | ÷ | 9 | × | a% |
| = | 21600 | + | 480 | a |
| 14600 | + | 146 | a | = | 21600 | + | 480 | a |
| 146 | a | − | 480 | a | = | 21600 | − | 14600 |
| - | 334 | a | = | 21600 | − | 14600 |
| - | 334 | a | = | 7000 |
| - | 7000 | = | 334 | a |
| 334 | a | = | - | 7000 |
| a | = | - | 7000 | ÷ | 334 |
| = | - | 7000 | × | 1 334 |
| = | - | 3500 | × | 1 167 |
| a | = | - | 3500 167 |
| a | = | - 20.958084 |