| ( | 60 | − | 50 | + | x | ) | ( | 800 | − | 2 | x | ) | = | 12000 |
| 方程左边 = | 60 | ( | 800 | − | 2 | x | ) | − | 50 | ( | 800 | − | 2 | x | ) | + | x | ( | 800 | − | 2 | x | ) |
| = | 60 | × | 800 | − | 60 | × | 2 | x | − | 50 | ( | 800 | − | 2 | x | ) | + | x | ( | 800 | − | 2 | x | ) |
| = | 48000 | − | 120 | x | − | 50 | ( | 800 | − | 2 | x | ) | + | x | ( | 800 | − | 2 | x | ) |
| = | 48000 | − | 120 | x | − | 50 | × | 800 | + | 50 | × | 2 | x | + | x | ( | 800 | − | 2 | x | ) |
| = | 48000 | − | 120 | x | − | 40000 | + | 100 | x | + | x | ( | 800 | − | 2 | x | ) |
| = | 8000 | − | 20 | x | + | x | ( | 800 | − | 2 | x | ) |
| = | 8000 | − | 20 | x | + | x | × | 800 | − | x | × | 2 | x |
| = | 8000 | + | 780 | x | − | x | × | 2 | x |
| 8000 | + | 780 | x | − | x | × | 2 | x | = | 12000 |