| ( | 100 | − | 3 | X | ) | ( | 80 | − | X | ) | = | 4900 |
| 方程左边 = | 100 | ( | 80 | − | X | ) | − | 3 | X | ( | 80 | − | X | ) |
| = | 100 | × | 80 | − | 100 | X | − | 3 | X | ( | 80 | − | X | ) |
| = | 8000 | − | 100 | X | − | 3 | X | ( | 80 | − | X | ) |
| = | 8000 | − | 100 | X | − | 3 | X | × | 80 | + | 3 | X | X |
| = | 8000 | − | 100 | X | − | 240 | X | + | 3 | X | X |
| = | 8000 | − | 340 | X | + | 3 | X | X |
| 8000 | − | 340 | X | + | 3 | X | X | = | 4900 |
| X2= | 310 3 |