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