| 4 | ( | 80 | − | t | ) | = | 20 | ( | t | − | 56 | ) |
| 方程左边 = | 4 | × | 80 | − | 4 | t |
| = | 320 | − | 4 | t |
| 320 | − | 4 | t | = | 20 | ( | t | − | 56 | ) |
| 方程右边 = | 20 | t | − | 20 | × | 56 |
| = | 20 | t | − | 1120 |
| 320 | − | 4 | t | = | 20 | t | − | 1120 |
| - | 4 | t | − | 20 | t | = | - | 1120 | − | 320 |
| - | 24 | t | = | - | 1120 | − | 320 |
| - | 24 | t | = | - | 1440 |
| 1440 | = | 24 | t |
| 24 | t | = | 1440 |
| t | = | 1440 | ÷ | 24 |
| = | 1440 | × | 1 24 |
| = | 60 | × | 1 |
| t | = | 60 |