| ( | 800 | − | 20 | t | ) | ( | 600 | − | 10 | t | ) | = | 30000 |
| 方程左边 = | 800 | ( | 600 | − | 10 | t | ) | − | 20 | t | ( | 600 | − | 10 | t | ) |
| = | 800 | × | 600 | − | 800 | × | 10 | t | − | 20 | t | ( | 600 | − | 10 | t | ) |
| = | 480000 | − | 8000 | t | − | 20 | t | ( | 600 | − | 10 | t | ) |
| = | 480000 | − | 8000 | t | − | 20 | t | × | 600 | + | 20 | t | × | 10 | t |
| = | 480000 | − | 8000 | t | − | 12000 | t | + | 200 | t | t |
| = | 480000 | − | 20000 | t | + | 200 | t | t |
| 480000 | − | 20000 | t | + | 200 | t | t | = | 30000 |