| ( | 90 | − | 30 | t | ) | ( | 90 | − | 30 | t | ) | + | 200 | t | t | = | 2500 |
| 方程左边 = | 90 | ( | 90 | − | 30 | t | ) | − | 30 | t | ( | 90 | − | 30 | t | ) | + | 200 | t | t |
| = | 90 | × | 90 | − | 90 | × | 30 | t | − | 30 | t | ( | 90 | − | 30 | t | ) | + | 200 | t | t |
| = | 8100 | − | 2700 | t | − | 30 | t | ( | 90 | − | 30 | t | ) | + | 200 | t | t |
| = | 8100 | − | 2700 | t | − | 30 | t | × | 90 | + | 30 | t | × | 30 | t | + | 200 | t |
| = | 8100 | − | 2700 | t | − | 2700 | t | + | 900 | t | t | + | 200 | t | t |
| = | 8100 | − | 5400 | t | + | 900 | t | t | + | 200 | t | t |
| 8100 | − | 5400 | t | + | 900 | t | t | + | 200 | t | t | = | 2500 |