| ( | 60 | x | − | 40 | ) | ( | 700 | − | 600 | x | ) | = | 2240 |
| 方程左边 = | 60 | x | ( | 700 | − | 600 | x | ) | − | 40 | ( | 700 | − | 600 | x | ) |
| = | 60 | x | × | 700 | − | 60 | x | × | 600 | x | − | 40 | ( | 700 | − | 600 | x | ) |
| = | 42000 | x | − | 36000 | x | x | − | 40 | ( | 700 | − | 600 | x | ) |
| = | 42000 | x | − | 36000 | x | x | − | 40 | × | 700 | + | 40 | × | 600 | x |
| = | 42000 | x | − | 36000 | x | x | − | 28000 | + | 24000 | x |
| = | 66000 | x | − | 36000 | x | x | − | 28000 |
| 66000 | x | − | 36000 | x | x | − | 28000 | = | 2240 |