| ( | 80 | − | 10 | x | ) | ( | 5 | x | − | 2 | ) | = | 1200 |
| 方程左边 = | 80 | ( | 5 | x | − | 2 | ) | − | 10 | x | ( | 5 | x | − | 2 | ) |
| = | 80 | × | 5 | x | − | 80 | × | 2 | − | 10 | x | ( | 5 | x | − | 2 | ) |
| = | 400 | x | − | 160 | − | 10 | x | ( | 5 | x | − | 2 | ) |
| = | 400 | x | − | 160 | − | 10 | x | × | 5 | x | + | 10 | x | × | 2 |
| = | 400 | x | − | 160 | − | 50 | x | x | + | 20 | x |
| = | 420 | x | − | 160 | − | 50 | x | x |
| 420 | x | − | 160 | − | 50 | x | x | = | 1200 |