| 20 | ÷ | 3 | = | a | × | 1 | ( | 1 | − | 1 | ÷ | 81 | ) | ÷ | 4 | ÷ | 3 |
| 方程左边 = | 20 3 |
20 3 | = | a | × | 1 | ( | 1 | − | 1 | ÷ | 81 | ) | ÷ | 4 | ÷ | 3 |
| 方程右边 = | a | × | 1 12 | ( | 1 | − | 1 | ÷ | 81 | ) |
20 3 | = | a | × | 1 12 | ( | 1 | − | 1 | ÷ | 81 | ) |
| 方程右边 = | a | × | 1 12 | × | 1 | − | a | × | 1 12 | × | 1 | ÷ | 81 |
| = | a | × | 1 12 | − | a | × | 1 972 |
| = | 20 243 | a |
20 3 | = | 20 243 | a |
| - | 20 243 | a | = | - | 20 3 |
20 3 | = | 20 243 | a |
20 243 | a | = | 20 3 |
| a | = | 20 3 | ÷ | 20 243 |
| = | 20 3 | × | 243 20 |
| = | 1 | × | 81 |
| a | = | 81 |