| 3 | ÷ | 4 | × | x | + | 4 | ÷ | 5 | × | ( | 2800 | − | x | ) | = | 2100 |
| 方程左边 = | 3 4 | x | + | 4 5 | ( | 2800 | − | x | ) |
3 4 | x | + | 4 5 | ( | 2800 | − | x | ) | = | 2100 |
| 方程左边 = | 3 4 | x | + | 4 5 | × | 2800 | − | 4 5 | x |
| = | 3 4 | x | + | 2240 | − | 4 5 | x |
| = | - | 1 20 | x | + | 2240 |
| - | 1 20 | x | + | 2240 | = | 2100 |
| - | 1 20 | x | = | 2100 | − | 2240 |
| - | 1 20 | x | = | - | 140 |
| 140 | = | 1 20 | x |
1 20 | x | = | 140 |
| x | = | 140 | ÷ | 1 20 |
| = | 140 | × | 20 |
| x | = | 2800 |