| ( | 21 4 | + | X | ) | ÷ | ( | 21 2 | + | X | ) | = | 3 5 |
| 方程两边同时乘以: | ( | 21 2 | + | X | ) |
| ( | 21 4 | + | X | ) | = | 3 5 | ( | 21 2 | + | X | ) |
21 4 | + | X | = | 3 5 | ( | 21 2 | + | X | ) |
21 4 | + | X | = | 3 5 | × | 21 2 | + | 3 5 | X |
21 4 | + | X | = | 63 10 | + | 3 5 | X |
| X | − | 3 5 | X | = | 63 10 | − | 21 4 |
2 5 | X | = | 63 10 | − | 21 4 |
2 5 | X | = | 21 20 |
| X | = | 21 20 | ÷ | 2 5 |
| = | 21 20 | × | 5 2 |
| = | 21 4 | × | 1 2 |
| X | = | 21 8 |
| X | = | 2.625 |