| 400 | = | ( | 20 | + | 2 | ( | 60 | − | x | ) | ) | ( | x | − | 40 | ) |
| 方程右边 = | 20 | ( | x | − | 40 | ) | + | 2 | ( | 60 | − | x | ) | ( | x | − | 40 | ) |
| = | 20 | x | − | 20 | × | 40 | + | 2 | ( | 60 | − | x | ) | ( | x | − | 40 | ) |
| = | 20 | x | − | 800 | + | 2 | ( | 60 | − | x | ) | ( | x | − | 40 | ) |
| = | 20 | x | − | 800 | + | 2 | × | 60 | ( | x | − | 40 | ) | − | 2 | x | ( | x | − | 40 | ) |
| = | 20 | x | − | 800 | + | 120 | ( | x | − | 40 | ) | − | 2 | x | ( | x | − | 40 | ) |
| = | 20 | x | − | 800 | + | 120 | x | − | 120 | × | 40 | − | 2 | x | ( | x | − | 40 | ) |
| = | 20 | x | − | 800 | + | 120 | x | − | 4800 | − | 2 | x | ( | x | − | 40 | ) |
| = | 140 | x | − | 5600 | − | 2 | x | ( | x | − | 40 | ) |
| = | 140 | x | − | 5600 | − | 2 | x | x | + | 2 | x | × | 40 |
| = | 140 | x | − | 5600 | − | 2 | x | x | + | 80 | x |
| = | 220 | x | − | 5600 | − | 2 | x | x |
| 400 | = | 220 | x | − | 5600 | − | 2 | x | x |