| ( | 120 | − | 80 | ) | × | 400 | + | ( | 500 | − | 400 | ) | ( | 120 | − | X | − | 80 | ) | = | 500 | × | 80 | × | 45 100 |
| 方程左边 = | 120 | × | 400 | − | 80 | × | 400 | + | ( | 500 | − | 400 | ) | ( | 120 | − | X | − | 80 | ) |
| = | 48000 | − | 32000 | + | ( | 500 | − | 400 | ) | ( | 120 | − | X | − | 80 | ) |
| = | 16000 | + | ( | 500 | − | 400 | ) | ( | 120 | − | X | − | 80 | ) |
| = | 16000 | + | 500 | ( | 120 | − | X | − | 80 | ) | − | 400 | ( | 120 | − | X | − | 80 | ) |
| = | 16000 | + | 500 | × | 120 | − | 500 | X | − | 500 | × | 80 | − | 400 | ( | 120 | − | X | − | 80 | ) |
| = | 16000 | + | 60000 | − | 500 | X | − | 40000 | − | 400 | ( | 120 | − | X | − | 80 | ) |
| = | 36000 | − | 500 | X | − | 400 | ( | 120 | − | X | − | 80 | ) |
| = | 36000 | − | 500 | X | − | 400 | × | 120 | + | 400 | X | + | 400 | × | 80 |
| = | 36000 | − | 500 | X | − | 48000 | + | 400 | X | + | 32000 |
| = | 20000 | − | 100 | X |
| 20000 | − | 100 | X | = | 500 | × | 80 | × | 45 100 |
| 方程右边 = | 18000 |
| 20000 | − | 100 | X | = | 18000 |
| - | 100 | X | = | 18000 | − | 20000 |
| - | 100 | X | = | - | 2000 |
| 2000 | = | 100 | X |
| 100 | X | = | 2000 |
| X | = | 2000 | ÷ | 100 |
| = | 2000 | × | 1 100 |
| = | 20 | × | 1 |
| X | = | 20 |