| 15 | x | ÷ | ( | 15 | x | − | 4200 | ) | = | 5 4 |
| 方程两边同时乘以: | ( | 15 | x | − | 4200 | ) |
| 15 | x | = | 5 4 | ( | 15 | x | − | 4200 | ) |
| 15 | x | = | 5 4 | × | 15 | x | − | 5 4 | × | 4200 |
| 15 | x | = | 75 4 | x | − | 5250 |
| 15 | x | − | 75 4 | x | = | - | 5250 |
| - | 15 4 | x | = | - | 5250 |
| 5250 | = | 15 4 | x |
15 4 | x | = | 5250 |
| x | = | 5250 | ÷ | 15 4 |
| = | 5250 | × | 4 15 |
| = | 350 | × | 4 |
| x | = | 1400 |