| ( | 80 | − | 4 | x | ) | ( | 20 | x | − | 20 | ) | = | 1080 |
| Left side of the equation = | 80 | ( | 20 | x | − | 20 | ) | − | 4 | x | ( | 20 | x | − | 20 | ) |
| = | 80 | × | 20 | x | − | 80 | × | 20 | − | 4 | x | ( | 20 | x | − | 20 | ) |
| = | 1600 | x | − | 1600 | − | 4 | x | ( | 20 | x | − | 20 | ) |
| = | 1600 | x | − | 1600 | − | 4 | x | × | 20 | x | + | 4 | x | × | 20 |
| = | 1600 | x | − | 1600 | − | 80 | x | x | + | 80 | x |
| = | 1680 | x | − | 1600 | − | 80 | x | x |
| 1680 | x | − | 1600 | − | 80 | x | x | = | 1080 |