| 80 | ( | 20 | ( | 1 | + | a% | ) | − | 15 | ) | + | 120 | ( | 30 | ( | 1 | − | a% | ) | − | ( | 20 | − | 3 | ) | ) | = | 1600 | + | 260 |
| 方程左边 = | 80 | × | 20 | ( | 1 | + | a% | ) | − | 80 | × | 15 | + | 120 | ( | 30 | ( | 1 | − | a% | ) | − | ( | 20 | − | 3 | ) | ) |
| = | 1600 | ( | 1 | + | a% | ) | − | 1200 | + | 120 | ( | 30 | ( | 1 | − | a% | ) | − | ( | 20 | − | 3 | ) | ) |
| = | 1600 | × | 1 | + | 1600 | × | a% | − | 1200 | + | 120 | ( | 30 | ( | 1 | − | a% | ) | − | ( | 20 | − | 3 | ) | ) |
| = | 1600 | + | 16 | a | − | 1200 | + | 120 | ( | 30 | ( | 1 | − | a% | ) | − | ( | 20 | − | 3 | ) | ) |
| = | 400 | + | 16 | a | + | 120 | ( | 30 | ( | 1 | − | a% | ) | − | ( | 20 | − | 3 | ) | ) |
| = | 400 | + | 16 | a | + | 120 | × | 30 | ( | 1 | − | a% | ) | − | 120 | ( | 20 | − | 3 | ) |
| = | 400 | + | 16 | a | + | 3600 | ( | 1 | − | a% | ) | − | 120 | ( | 20 | − | 3 | ) |
| = | 400 | + | 16 | a | + | 3600 | × | 1 | − | 3600 | × | a% | − | 120 | ( | 20 | − | 3 | ) |
| = | 400 | + | 16 | a | + | 3600 | − | 36 | a | − | 120 | ( | 20 | − | 3 | ) |
| = | 4000 | − | 20 | a | − | 120 | ( | 20 | − | 3 | ) |
| = | 4000 | − | 20 | a | − | 120 | × | 20 | + | 120 | × | 3 |
| = | 4000 | − | 20 | a | − | 2400 | + | 360 |
| = | 1960 | − | 20 | a |
| 1960 | − | 20 | a | = | 1600 | + | 260 |
| 方程右边 = | 1860 |
| 1960 | − | 20 | a | = | 1860 |
| - | 20 | a | = | 1860 | − | 1960 |
| - | 20 | a | = | - | 100 |
| 100 | = | 20 | a |
| 20 | a | = | 100 |
| a | = | 100 | ÷ | 20 |
| = | 100 | × | 1 20 |
| = | 5 | × | 1 |
| a | = | 5 |