| ( | 19 | − | y | ) | ( | 3000 | + | 30 | y | ) | = | 6300 |
| 方程左边 = | 19 | ( | 3000 | + | 30 | y | ) | − | y | ( | 3000 | + | 30 | y | ) |
| = | 19 | × | 3000 | + | 19 | × | 30 | y | − | y | ( | 3000 | + | 30 | y | ) |
| = | 57000 | + | 570 | y | − | y | ( | 3000 | + | 30 | y | ) |
| = | 57000 | + | 570 | y | − | y | × | 3000 | − | y | × | 30 | y |
| = | 57000 | − | 2430 | y | − | y | × | 30 | y |
| 57000 | − | 2430 | y | − | y | × | 30 | y | = | 6300 |