| ( | 100 | − | t | ) | × | 210 | × | 9 20 | = | t | × | 90 | × | 21 5 |
| 方程左边 = | ( | 100 | − | t | ) | × | 189 2 |
| ( | 100 | − | t | ) | × | 189 2 | = | t | × | 90 | × | 21 5 |
| 方程左边 = | 100 | × | 189 2 | − | t | × | 189 2 |
| = | 9450 | − | t | × | 189 2 |
| 9450 | − | 189 2 | t | = | t | × | 90 | × | 21 5 |
| 方程右边 = | t | × | 378 |
| 9450 | − | 189 2 | t | = | 378 | t |
| - | 189 2 | t | − | 378 | t | = | - | 9450 |
| - | 945 2 | t | = | - | 9450 |
| 9450 | = | 945 2 | t |
945 2 | t | = | 9450 |
| t | = | 9450 | ÷ | 945 2 |
| = | 9450 | × | 2 945 |
| = | 10 | × | 2 |
| t | = | 20 |