| ( | 80 | − | 20 | a | ) | ( | 10 | + | a | ) | = | 12000 |
| 方程左边 = | 80 | ( | 10 | + | a | ) | − | 20 | a | ( | 10 | + | a | ) |
| = | 80 | × | 10 | + | 80 | a | − | 20 | a | ( | 10 | + | a | ) |
| = | 800 | + | 80 | a | − | 20 | a | ( | 10 | + | a | ) |
| = | 800 | + | 80 | a | − | 20 | a | × | 10 | − | 20 | a | a |
| = | 800 | + | 80 | a | − | 200 | a | − | 20 | a | a |
| = | 800 | − | 120 | a | − | 20 | a | a |
| 800 | − | 120 | a | − | 20 | a | a | = | 12000 |