| ( | 60 | − | x | − | 40 | ) | ( | 100 | + | 20 | x | ) | = | 2240 |
| 方程左边 = | 60 | ( | 100 | + | 20 | x | ) | − | x | ( | 100 | + | 20 | x | ) | − | 40 | ( | 100 | + | 20 | x | ) |
| = | 60 | × | 100 | + | 60 | × | 20 | x | − | x | ( | 100 | + | 20 | x | ) | − | 40 | ( | 100 | + | 20 | x | ) |
| = | 6000 | + | 1200 | x | − | x | ( | 100 | + | 20 | x | ) | − | 40 | ( | 100 | + | 20 | x | ) |
| = | 6000 | + | 1200 | x | − | x | × | 100 | − | x | × | 20 | x | − | 40 | ( | 100 | + | 20 | x | ) |
| = | 6000 | + | 1100 | x | − | x | × | 20 | x | − | 40 | ( | 100 | + | 20 | x | ) |
| = | 6000 | + | 1100 | x | − | x | × | 20 | x | − | 40 | × | 100 | − | 40 | × | 20 | x |
| = | 6000 | + | 1100 | x | − | x | × | 20 | x | − | 4000 | − | 800 | x |
| = | 2000 | + | 300 | x | − | x | × | 20 | x |
| 2000 | + | 300 | x | − | x | × | 20 | x | = | 2240 |