| ( | y | − | 6 | ) | ( | 300 | + | 750 | − | 30 | y | ) | = | 6300 |
| 方程左边 = | y | ( | 300 | + | 750 | − | 30 | y | ) | − | 6 | ( | 300 | + | 750 | − | 30 | y | ) |
| = | y | × | 300 | + | y | × | 750 | − | y | × | 30 | y | − | 6 | ( | 300 | + | 750 | − | 30 | y | ) |
| = | 1050 | y | − | y | × | 30 | y | − | 6 | ( | 300 | + | 750 | − | 30 | y | ) |
| = | 1050 | y | − | y | × | 30 | y | − | 6 | × | 300 | − | 6 | × | 750 | + | 6 | × | 30 | y |
| = | 1050 | y | − | y | × | 30 | y | − | 1800 | − | 4500 | + | 180 | y |
| = | 1230 | y | − | y | × | 30 | y | − | 6300 |
| 1230 | y | − | y | × | 30 | y | − | 6300 | = | 6300 |