| ( | 21 | ÷ | 5 | ) | ÷ | x | = | ( | 2 | ÷ | 5 | ) | − | ( | 13 | ÷ | 8 | ) |
| 方程两边同时乘以: | x |
| ( | 21 | ÷ | 5 | ) | = | ( | 2 | ÷ | 5 | ) | x | − | ( | 13 | ÷ | 8 | ) | x |
| 21 | ÷ | 5 | = | ( | 2 | ÷ | 5 | ) | x | − | ( | 13 | ÷ | 8 | ) | x |
| 21 | ÷ | 5 | = | 2 | ÷ | 5 | × | x | − | ( | 13 | ÷ | 8 | ) | x |
21 5 | = | 2 5 | x | − | ( | 13 | ÷ | 8 | ) | x |
21 5 | = | 2 5 | x | − | 13 | ÷ | 8 | × | x |
21 5 | = | 2 5 | x | − | 13 8 | x |
21 5 | = | - | 49 40 | x |
49 40 | x | = | - | 21 5 |
| x | = | - | 21 5 | ÷ | 49 40 |
| = | - | 21 5 | × | 40 49 |
| = | - | 3 | × | 8 7 |
| x | = | - | 24 7 |
| x | = | - 3.428571 |