| 12 | − | 3 | ( | 9 | − | y | ) | = | 5 | ( | y | − | 4 | ) |
| 方程左边 = | 12 | − | 3 | × | 9 | + | 3 | y |
| = | 12 | − | 27 | + | 3 | y |
| = | - | 15 | + | 3 | y |
| - | 15 | + | 3 | y | = | 5 | ( | y | − | 4 | ) |
| 方程右边 = | 5 | y | − | 5 | × | 4 |
| = | 5 | y | − | 20 |
| - | 15 | + | 3 | y | = | 5 | y | − | 20 |
| 3 | y | − | 5 | y | = | - | 20 | + | 15 |
| - | 2 | y | = | - | 20 | + | 15 |
| - | 2 | y | = | - | 5 |
| 5 | = | 2 | y |
| 2 | y | = | 5 |
| y | = | 5 | ÷ | 2 |
| = | 5 | × | 1 2 |
| y | = | 5 2 |
| y | = | 2.5 |