| ( | 27000 | − | P | ) | ( | 9 20 | − | 1 2 | ) | = | 0 |
| 方程左边 = | 27000 | ( | 9 20 | − | 1 2 | ) | − | P | ( | 9 20 | − | 1 2 | ) |
| = | 27000 | × | 9 20 | − | 27000 | × | 1 2 | − | P | ( | 9 20 | − | 1 2 | ) |
| = | 12150 | − | 13500 | − | P | ( | 9 20 | − | 1 2 | ) |
| = | - | 1350 | − | P | ( | 9 20 | − | 1 2 | ) |
| = | - | 1350 | − | P | × | 9 20 | + | P | × | 1 2 |
| = | - | 1350 | + | 1 20 | P |
| - | 1350 | + | 1 20 | P | = | 0 |
1 20 | P | = | 0 | + | 1350 |
1 20 | P | = | 1350 |
| P | = | 1350 | ÷ | 1 20 |
| = | 1350 | × | 20 |
| P | = | 27000 |