| ( | 15 | − | 1 10 | a | ) | ( | a | − | 20 | ) | = | 300 |
| 方程左边 = | 15 | ( | a | − | 20 | ) | − | 1 10 | a | ( | a | − | 20 | ) |
| = | 15 | a | − | 15 | × | 20 | − | 1 10 | a | ( | a | − | 20 | ) |
| = | 15 | a | − | 300 | − | 1 10 | a | ( | a | − | 20 | ) |
| = | 15 | a | − | 300 | − | 1 10 | a | a | + | 1 10 | a | × | 20 |
| = | 15 | a | − | 300 | − | 1 10 | a | a | + | 2 | a |
| = | 17 | a | − | 300 | − | 1 10 | a | a |
| 17 | a | − | 300 | − | 1 10 | a | a | = | 300 |