| ( | 20 | − | 2 | x | ) | ( | 200 | + | 20 | x | ) | = | 2240 |
| 方程左边 = | 20 | ( | 200 | + | 20 | x | ) | − | 2 | x | ( | 200 | + | 20 | x | ) |
| = | 20 | × | 200 | + | 20 | × | 20 | x | − | 2 | x | ( | 200 | + | 20 | x | ) |
| = | 4000 | + | 400 | x | − | 2 | x | ( | 200 | + | 20 | x | ) |
| = | 4000 | + | 400 | x | − | 2 | x | × | 200 | − | 2 | x | × | 20 | x |
| = | 4000 | + | 400 | x | − | 400 | x | − | 40 | x | x |
| = | 4000 | + | 0 | x | − | 40 | x | x |
| 4000 | − | 40 | x | x | = | 2240 |