| ( | 60 | − | 2 | X | ) | ( | 40 | − | 2 | X | ) | = | 1500 |
| 方程左边 = | 60 | ( | 40 | − | 2 | X | ) | − | 2 | X | ( | 40 | − | 2 | X | ) |
| = | 60 | × | 40 | − | 60 | × | 2 | X | − | 2 | X | ( | 40 | − | 2 | X | ) |
| = | 2400 | − | 120 | X | − | 2 | X | ( | 40 | − | 2 | X | ) |
| = | 2400 | − | 120 | X | − | 2 | X | × | 40 | + | 2 | X | × | 2 | X |
| = | 2400 | − | 120 | X | − | 80 | X | + | 4 | X | X |
| = | 2400 | − | 200 | X | + | 4 | X | X |
| 2400 | − | 200 | X | + | 4 | X | X | = | 1500 |