| 5 | × | 42 | + | ( | a | − | 5 | ) | × | 42 | × | 4 5 | = | 182 5 | a |
| 方程左边 = | 210 | + | ( | a | − | 5 | ) | × | 168 5 |
| 210 | + | ( | a | − | 5 | ) | × | 168 5 | = | 182 5 | a |
| 方程左边 = | 210 | + | a | × | 168 5 | − | 5 | × | 168 5 |
| = | 210 | + | a | × | 168 5 | − | 168 |
| = | 42 | + | 168 5 | a |
| 42 | + | 168 5 | a | = | 182 5 | a |
168 5 | a | − | 182 5 | a | = | - | 42 |
| - | 14 5 | a | = | - | 42 |
| 42 | = | 14 5 | a |
14 5 | a | = | 42 |
| a | = | 42 | ÷ | 14 5 |
| = | 42 | × | 5 14 |
| = | 3 | × | 5 |
| a | = | 15 |