| ( | 800 | − | a | ) | × | 7 | + | 4000 | = | ( | 800 | − | a | ) | × | 5 | + | 5000 |
| 方程左边 = | 800 | × | 7 | − | a | × | 7 | + | 4000 |
| = | 5600 | − | a | × | 7 | + | 4000 |
| = | 9600 | − | 7 | a |
| 9600 | − | 7 | a | = | ( | 800 | − | a | ) | × | 5 | + | 5000 |
| 方程右边 = | 800 | × | 5 | − | a | × | 5 | + | 5000 |
| = | 4000 | − | a | × | 5 | + | 5000 |
| = | 9000 | − | 5 | a |
| 9600 | − | 7 | a | = | 9000 | − | 5 | a |
| - | 7 | a | + | 5 | a | = | 9000 | − | 9600 |
| - | 2 | a | = | 9000 | − | 9600 |
| - | 2 | a | = | - | 600 |
| 600 | = | 2 | a |
| 2 | a | = | 600 |
| a | = | 600 | ÷ | 2 |
| = | 600 | × | 1 2 |
| = | 300 | × | 1 |
| a | = | 300 |