| 420 | = | 120 | + | 2 | × | 10 | × | 2 | + | 3 | × | 13 | ( | x | − | 1 2 | ) |
| 方程右边 = | 120 | + | 40 | + | 39 | ( | x | − | 1 2 | ) |
| = | 160 | + | 39 | ( | x | − | 1 2 | ) |
| 420 | = | 160 | + | 39 | ( | x | − | 1 2 | ) |
| 方程右边 = | 160 | + | 39 | x | − | 39 | × | 1 2 |
| = | 160 | + | 39 | x | − | 39 2 |
| = | 281 2 | + | 39 | x |
| 420 | = | 281 2 | + | 39 | x |
| - | 39 | x | = | 281 2 | − | 420 |
| - | 39 | x | = | - | 559 2 |
559 2 | = | 39 | x |
| 39 | x | = | 559 2 |
| x | = | 559 2 | ÷ | 39 |
| = | 559 2 | × | 1 39 |
| = | 43 2 | × | 1 3 |
| x | = | 43 6 |
| x | = | 7.166667 |