| ( | 80 | − | 1 2 | X | ) | ( | 40 | − | 1 2 | X | ) | = | 400 |
| 方程左边 = | 80 | ( | 40 | − | 1 2 | X | ) | − | 1 2 | X | ( | 40 | − | 1 2 | X | ) |
| = | 80 | × | 40 | − | 80 | × | 1 2 | X | − | 1 2 | X | ( | 40 | − | 1 2 | X | ) |
| = | 3200 | − | 40 | X | − | 1 2 | X | ( | 40 | − | 1 2 | X | ) |
| = | 3200 | − | 40 | X | − | 1 2 | X | × | 40 | + | 1 2 | X | × | 1 2 | X |
| = | 3200 | − | 40 | X | − | 20 | X | + | 1 4 | X | X |
| = | 3200 | − | 60 | X | + | 1 4 | X | X |
| 3200 | − | 60 | X | + | 1 4 | X | X | = | 400 |