| ( | 800 | − | 20 | a | ) | ( | 10 | + | a | ) | = | 12000 |
| 方程左边 = | 800 | ( | 10 | + | a | ) | − | 20 | a | ( | 10 | + | a | ) |
| = | 800 | × | 10 | + | 800 | a | − | 20 | a | ( | 10 | + | a | ) |
| = | 8000 | + | 800 | a | − | 20 | a | ( | 10 | + | a | ) |
| = | 8000 | + | 800 | a | − | 20 | a | × | 10 | − | 20 | a | a |
| = | 8000 | + | 800 | a | − | 200 | a | − | 20 | a | a |
| = | 8000 | + | 600 | a | − | 20 | a | a |
| 8000 | + | 600 | a | − | 20 | a | a | = | 12000 |