| ( | x | − | 80 | ) | ( | ( | 100 | − | x | ) | × | 5 | + | 50 | ) | = | 1080 |
| 方程左边 = | x | ( | ( | 100 | − | x | ) | × | 5 | + | 50 | ) | − | 80 | ( | ( | 100 | − | x | ) | × | 5 | + | 50 | ) |
| = | x | ( | 100 | − | x | ) | × | 5 | + | x | × | 50 | − | 80 | ( | ( | 100 | − | x | ) | × | 5 | + | 50 | ) |
| = | x | × | 100 | × | 5 | − | x | x | × | 5 | + | 50 | x | − | 80 | ( | ( | 100 | − | x | ) | × | 5 | + | 50 | ) |
| = | x | × | 500 | − | x | x | × | 5 | + | 50 | x | − | 80 | ( | ( | 100 | − | x | ) | × | 5 | + | 50 | ) |
| = | 550 | x | − | x | x | × | 5 | − | 80 | ( | ( | 100 | − | x | ) | × | 5 | + | 50 | ) |
| = | 550 | x | − | x | x | × | 5 | − | 80 | ( | 100 | − | x | ) | × | 5 | − | 80 | × | 50 |
| = | 550 | x | − | x | x | × | 5 | − | 400 | ( | 100 | − | x | ) | − | 4000 |
| = | 550 | x | − | x | x | × | 5 | − | 400 | × | 100 | + | 400 | x | − | 4000 |
| = | 550 | x | − | x | x | × | 5 | − | 40000 | + | 400 | x | − | 4000 |
| = | 950 | x | − | x | x | × | 5 | − | 44000 |
| 950 | x | − | x | x | × | 5 | − | 44000 | = | 1080 |