| ( | 20977 5 | + | 20 | y | ) | ÷ | ( | 100 | + | y | ) | = | 21 |
| 方程两边同时乘以: | ( | 100 | + | y | ) |
| ( | 20977 5 | + | 20 | y | ) | = | 21 | ( | 100 | + | y | ) |
20977 5 | + | 20 | y | = | 21 | ( | 100 | + | y | ) |
20977 5 | + | 20 | y | = | 21 | × | 100 | + | 21 | y |
20977 5 | + | 20 | y | = | 2100 | + | 21 | y |
| 20 | y | − | 21 | y | = | 2100 | − | 20977 5 |
| - | y | = | 2100 | − | 20977 5 |
| - | y | = | 2100 | − | 20977 5 |
| - | y | = | - | 10477 5 |
10477 5 | = | y |
| y | = | 10477 5 |