| ( | x | − | 400 | ) | × | 3 4 | ÷ | 320 | = | ( | x | − | 640 | ) | × | 3 4 | ÷ | 240 |
| 方程左边 = | ( | x | − | 400 | ) | × | 3 1280 |
| ( | x | − | 400 | ) | × | 3 1280 | = | ( | x | − | 640 | ) | × | 3 4 | ÷ | 240 |
| 方程左边 = | x | × | 3 1280 | − | 400 | × | 3 1280 |
| = | x | × | 3 1280 | − | 15 16 |
3 1280 | x | − | 15 16 | = | ( | x | − | 640 | ) | × | 3 4 | ÷ | 240 |
| 方程右边 = | ( | x | − | 640 | ) | × | 1 320 |
3 1280 | x | − | 15 16 | = | ( | x | − | 640 | ) | × | 1 320 |
| 方程右边 = | x | × | 1 320 | − | 640 | × | 1 320 |
| = | x | × | 1 320 | − | 2 |
3 1280 | x | − | 15 16 | = | 1 320 | x | − | 2 |
3 1280 | x | − | 1 320 | x | = | - | 2 | + | 15 16 |
| - | 1 1280 | x | = | - | 2 | + | 15 16 |
| - | 1 1280 | x | = | - | 17 16 |
17 16 | = | 1 1280 | x |
1 1280 | x | = | 17 16 |
| x | = | 17 16 | ÷ | 1 1280 |
| = | 17 16 | × | 1280 |
| = | 17 | × | 80 |
| x | = | 1360 |