| ( | ( | 60 | − | x | ) | × | 20 | + | 40 | ) | ( | 60 | − | x | ) | = | 1440 |
| 方程左边 = | ( | 60 | − | x | ) | × | 20 | ( | 60 | − | x | ) | + | 40 | ( | 60 | − | x | ) |
| = | 60 | × | 20 | ( | 60 | − | x | ) | − | x | × | 20 | ( | 60 | − | x | ) | + | 40 | ( | 60 | − | x | ) |
| = | 1200 | ( | 60 | − | x | ) | − | x | × | 20 | ( | 60 | − | x | ) | + | 40 | ( | 60 | − | x | ) |
| = | 1200 | × | 60 | − | 1200 | x | − | x | × | 20 | ( | 60 | − | x | ) | + | 40 | ( | 60 | − | x | ) |
| = | 72000 | − | 1200 | x | − | x | × | 20 | ( | 60 | − | x | ) | + | 40 | ( | 60 | − | x | ) |
| = | 72000 | − | 1200 | x | − | x | × | 20 | × | 60 | + | x | × | 20 | x | + | 40 | ( | 60 | − | x | ) |
| = | 72000 | − | 1200 | x | − | x | × | 1200 | + | x | × | 20 | x | + | 40 | ( | 60 | − | x | ) |
| = | 72000 | − | 2400 | x | + | x | × | 20 | x | + | 40 | ( | 60 | − | x | ) |
| = | 72000 | − | 2400 | x | + | x | × | 20 | x | + | 40 | × | 60 | − | 40 | x |
| = | 72000 | − | 2400 | x | + | x | × | 20 | x | + | 2400 | − | 40 | x |
| = | 74400 | − | 2440 | x | + | x | × | 20 | x |
| 74400 | − | 2440 | x | + | x | × | 20 | x | = | 1440 |