| 80 | × | 18 | = | ( | 50 | + | 2 | x | ) | ( | 28 | − | x | ) |
| 方程左边 = | 1440 |
| 1440 | = | ( | 50 | + | 2 | x | ) | ( | 28 | − | x | ) |
| 方程右边 = | 50 | ( | 28 | − | x | ) | + | 2 | x | ( | 28 | − | x | ) |
| = | 50 | × | 28 | − | 50 | x | + | 2 | x | ( | 28 | − | x | ) |
| = | 1400 | − | 50 | x | + | 2 | x | ( | 28 | − | x | ) |
| = | 1400 | − | 50 | x | + | 2 | x | × | 28 | − | 2 | x | x |
| = | 1400 | − | 50 | x | + | 56 | x | − | 2 | x | x |
| = | 1400 | + | 6 | x | − | 2 | x | x |
| 1440 | = | 1400 | + | 6 | x | − | 2 | x | x |