| ( | 10 | + | y | ) | ( | 500 | − | 40 | y | ) | = | 4500 |
| 方程左边 = | 10 | ( | 500 | − | 40 | y | ) | + | y | ( | 500 | − | 40 | y | ) |
| = | 10 | × | 500 | − | 10 | × | 40 | y | + | y | ( | 500 | − | 40 | y | ) |
| = | 5000 | − | 400 | y | + | y | ( | 500 | − | 40 | y | ) |
| = | 5000 | − | 400 | y | + | y | × | 500 | − | y | × | 40 | y |
| = | 5000 | + | 100 | y | − | y | × | 40 | y |
| 5000 | + | 100 | y | − | y | × | 40 | y | = | 4500 |
| y1= | - | 5 2 |
| y2=5 |