| 30 | ( | ( | 5 4 | X | + | 15 | ) | X | − | 2000 | ) | + | 100 | ( | 5 4 | X | + | 15 | ) | X | = | 642000 |
| 方程左边 = | 30 | ( | 5 4 | X | + | 15 | ) | X | − | 30 | × | 2000 | + | 100 | ( | 5 4 | X | + | 15 | ) | X |
| = | 30 | ( | 5 4 | X | + | 15 | ) | X | − | 60000 | + | 100 | ( | 5 4 | X | + | 15 | ) | X |
| = | 30 | × | 5 4 | X | X | + | 30 | × | 15 | X | − | 60000 | + | 100 | ( | 5 4 | X | + | 15 | ) | X |
| = | 75 2 | X | X | + | 450 | X | − | 60000 | + | 100 | ( | 5 4 | X | + | 15 | ) | X |
| = | 75 2 | X | X | + | 450 | X | − | 60000 | + | 100 | × | 5 4 | X | X | + | 100 | × | 15 |
| = | 75 2 | X | X | + | 450 | X | − | 60000 | + | 125 | X | X | + | 1500 | X |
| = | 75 2 | X | X | + | 1950 | X | − | 60000 | + | 125 | X | X |
75 2 | X | X | + | 1950 | X | − | 60000 | + | 125 | X | X | = | 642000 |