| ( | a | − | 250 | ) | ( | - | 4 | a | ÷ | 5 | + | 48 | ) | − | 80 | = | 640 |
| 方程左边 = | a | ( | - | 4 | a | ÷ | 5 | + | 48 | ) | − | 250 | ( | - | 4 | a | ÷ | 5 | + | 48 | ) | − | 80 |
| = | - | a | × | 4 | a | ÷ | 5 | + | a | × | 48 | − | 250 | ( | - | 4 | a | ÷ | 5 | + | 48 | ) | − | 80 |
| = | - | a | × | 4 5 | a | + | a | × | 48 | − | 250 | ( | - | 4 | a | ÷ | 5 | + | 48 | ) | − | 80 |
| = | - | a | × | 4 5 | a | + | 48 | a | + | 250 | × | 4 | a | ÷ | 5 | − | 250 | × | 48 | − | 80 |
| = | - | a | × | 4 5 | a | + | 48 | a | + | 200 | a | − | 12000 | − | 80 |
| = | - | a | × | 4 5 | a | + | 248 | a | − | 12080 |
| - | a | × | 4 5 | a | + | 248 | a | − | 12080 | = | 640 |