| 30 | × | 69 | + | 75 | x | + | 100 | ( | 70 | − | x | ) | = | 431 5 | × | 100 |
| 方程左边 = | 2070 | + | 75 | x | + | 100 | ( | 70 | − | x | ) |
| 2070 | + | 75 | x | + | 100 | ( | 70 | − | x | ) | = | 431 5 | × | 100 |
| 方程左边 = | 2070 | + | 75 | x | + | 100 | × | 70 | − | 100 | x |
| = | 2070 | + | 75 | x | + | 7000 | − | 100 | x |
| = | 9070 | − | 25 | x |
| 9070 | − | 25 | x | = | 431 5 | × | 100 |
| 方程右边 = | 8620 |
| 9070 | − | 25 | x | = | 8620 |
| - | 25 | x | = | 8620 | − | 9070 |
| - | 25 | x | = | - | 450 |
| 450 | = | 25 | x |
| 25 | x | = | 450 |
| x | = | 450 | ÷ | 25 |
| = | 450 | × | 1 25 |
| = | 18 | × | 1 |
| x | = | 18 |