| 4000 | − | ( | 600 | × | 75 100 | − | 400 | ) | × | 30 | = | ( | y | − | 75 | − | 650 | ) | × | 20 |
| 方程左边 = | 4000 | − | 600 | × | 75 100 | × | 30 | + | 400 | × | 30 |
| = | 4000 | − | 13500 | + | 12000 |
| = | 2500 |
| 2500 | = | ( | y | − | 75 | − | 650 | ) | × | 20 |
| 方程右边 = | y | × | 20 | − | 75 | × | 20 | − | 650 | × | 20 |
| = | y | × | 20 | − | 1500 | − | 13000 |
| = | 20 | y | − | 14500 |
| 2500 | = | 20 | y | − | 14500 |
| - | 20 | y | = | - | 14500 | − | 2500 |
| - | 20 | y | = | - | 17000 |
| 17000 | = | 20 | y |
| 20 | y | = | 17000 |
| y | = | 17000 | ÷ | 20 |
| = | 17000 | × | 1 20 |
| = | 850 | × | 1 |
| y | = | 850 |