| ( | 30 | − | 2 | t | ) | ( | 15 | − | 2 | t | ) | = | 200 |
| 方程左边 = | 30 | ( | 15 | − | 2 | t | ) | − | 2 | t | ( | 15 | − | 2 | t | ) |
| = | 30 | × | 15 | − | 30 | × | 2 | t | − | 2 | t | ( | 15 | − | 2 | t | ) |
| = | 450 | − | 60 | t | − | 2 | t | ( | 15 | − | 2 | t | ) |
| = | 450 | − | 60 | t | − | 2 | t | × | 15 | + | 2 | t | × | 2 | t |
| = | 450 | − | 60 | t | − | 30 | t | + | 4 | t | t |
| = | 450 | − | 90 | t | + | 4 | t | t |
| 450 | − | 90 | t | + | 4 | t | t | = | 200 |