| 2 | t | = | 2 | ( | 2 | t | − | ( | 30 | − | 3 | t | ) | ) |
| 方程右边 = | 2 | × | 2 | t | − | 2 | ( | 30 | − | 3 | t | ) |
| = | 4 | t | − | 2 | ( | 30 | − | 3 | t | ) |
| = | 4 | t | − | 2 | × | 30 | + | 2 | × | 3 | t |
| = | 4 | t | − | 60 | + | 6 | t |
| = | 10 | t | − | 60 |
| 2 | t | = | 10 | t | − | 60 |
| 2 | t | − | 10 | t | = | - | 60 |
| - | 8 | t | = | - | 60 |
| 60 | = | 8 | t |
| 8 | t | = | 60 |
| t | = | 60 | ÷ | 8 |
| = | 60 | × | 1 8 |
| = | 15 | × | 1 2 |
| t | = | 15 2 |
| t | = | 7.5 |