| ( | 20 | − | x | ) | ( | 100 | + | 20 | x | ) | = | 2280 |
| Left side of the equation = | 20 | ( | 100 | + | 20 | x | ) | − | x | ( | 100 | + | 20 | x | ) |
| = | 20 | × | 100 | + | 20 | × | 20 | x | − | x | ( | 100 | + | 20 | x | ) |
| = | 2000 | + | 400 | x | − | x | ( | 100 | + | 20 | x | ) |
| = | 2000 | + | 400 | x | − | x | × | 100 | − | x | × | 20 | x |
| = | 2000 | + | 300 | x | − | x | × | 20 | x |
| 2000 | + | 300 | x | − | x | × | 20 | x | = | 2280 |