| ( | 10 | + | a | ) | ÷ | ( | 100 | + | 3 | a | ) | = | 20 100 |
| 方程两边同时乘以: | ( | 100 | + | 3 | a | ) |
| ( | 10 | + | a | ) | = | 20 100 | ( | 100 | + | 3 | a | ) |
| 10 | + | a | = | 20 100 | ( | 100 | + | 3 | a | ) |
| 10 | + | a | = | 20 100 | × | 100 | + | 20 100 | × | 3 | a |
| 10 | + | a | = | 20 | + | 3 5 | a |
| a | − | 3 5 | a | = | 20 | − | 10 |
2 5 | a | = | 20 | − | 10 |
2 5 | a | = | 10 |
| a | = | 10 | ÷ | 2 5 |
| = | 10 | × | 5 2 |
| = | 5 | × | 5 |
| a | = | 25 |