| ( | 100 | − | 80 | − | x | ) | ( | 100 | + | 10 | x | ) | = | 2160 |
| 方程左边 = | 100 | ( | 100 | + | 10 | x | ) | − | 80 | ( | 100 | + | 10 | x | ) | − | x | ( | 100 | + | 10 | x | ) |
| = | 100 | × | 100 | + | 100 | × | 10 | x | − | 80 | ( | 100 | + | 10 | x | ) | − | x | ( | 100 | + | 10 | x | ) |
| = | 10000 | + | 1000 | x | − | 80 | ( | 100 | + | 10 | x | ) | − | x | ( | 100 | + | 10 | x | ) |
| = | 10000 | + | 1000 | x | − | 80 | × | 100 | − | 80 | × | 10 | x | − | x | ( | 100 | + | 10 | x | ) |
| = | 10000 | + | 1000 | x | − | 8000 | − | 800 | x | − | x | ( | 100 | + | 10 | x | ) |
| = | 2000 | + | 200 | x | − | x | ( | 100 | + | 10 | x | ) |
| = | 2000 | + | 200 | x | − | x | × | 100 | − | x | × | 10 | x |
| = | 2000 | + | 100 | x | − | x | × | 10 | x |
| 2000 | + | 100 | x | − | x | × | 10 | x | = | 2160 |