| ( | 20 | − | 6 | X | ) | ( | 30 | − | 4 | X | ) | = | 150 |
| 方程左边 = | 20 | ( | 30 | − | 4 | X | ) | − | 6 | X | ( | 30 | − | 4 | X | ) |
| = | 20 | × | 30 | − | 20 | × | 4 | X | − | 6 | X | ( | 30 | − | 4 | X | ) |
| = | 600 | − | 80 | X | − | 6 | X | ( | 30 | − | 4 | X | ) |
| = | 600 | − | 80 | X | − | 6 | X | × | 30 | + | 6 | X | × | 4 | X |
| = | 600 | − | 80 | X | − | 180 | X | + | 24 | X | X |
| = | 600 | − | 260 | X | + | 24 | X | X |
| 600 | − | 260 | X | + | 24 | X | X | = | 150 |