| 180 | × | 11 20 | + | ( | 280 | − | 180 | ) | ( | 11 20 | + | a | ) | = | 164 |
| 方程左边 = | 99 | + | ( | 280 | − | 180 | ) | ( | 11 20 | + | a | ) |
| 99 | + | ( | 280 | − | 180 | ) | ( | 11 20 | + | a | ) | = | 164 |
| 方程左边 = | 99 | + | 280 | ( | 11 20 | + | a | ) | − | 180 | ( | 11 20 | + | a | ) |
| = | 99 | + | 280 | × | 11 20 | + | 280 | a | − | 180 | ( | 11 20 | + | a | ) |
| = | 99 | + | 154 | + | 280 | a | − | 180 | ( | 11 20 | + | a | ) |
| = | 253 | + | 280 | a | − | 180 | ( | 11 20 | + | a | ) |
| = | 253 | + | 280 | a | − | 180 | × | 11 20 | − | 180 | a |
| = | 253 | + | 280 | a | − | 99 | − | 180 | a |
| = | 154 | + | 100 | a |
| 154 | + | 100 | a | = | 164 |
| 100 | a | = | 164 | − | 154 |
| 100 | a | = | 10 |
| a | = | 10 | ÷ | 100 |
| = | 10 | × | 1 100 |
| = | 1 | × | 1 10 |
| a | = | 1 10 |
| a | = | 0.1 |