| ( | 800 | − | 10 | x | ) | ( | x | − | 20 | ) | = | 8000 |
| Left side of the equation = | 800 | ( | x | − | 20 | ) | − | 10 | x | ( | x | − | 20 | ) |
| = | 800 | x | − | 800 | × | 20 | − | 10 | x | ( | x | − | 20 | ) |
| = | 800 | x | − | 16000 | − | 10 | x | ( | x | − | 20 | ) |
| = | 800 | x | − | 16000 | − | 10 | x | x | + | 10 | x | × | 20 |
| = | 800 | x | − | 16000 | − | 10 | x | x | + | 200 | x |
| = | 1000 | x | − | 16000 | − | 10 | x | x |
| 1000 | x | − | 16000 | − | 10 | x | x | = | 8000 |