| ( | 80 | − | x | ) | ( | 200 | + | 10 | x | ) | = | 9000 |
| Left side of the equation = | 80 | ( | 200 | + | 10 | x | ) | − | x | ( | 200 | + | 10 | x | ) |
| = | 80 | × | 200 | + | 80 | × | 10 | x | − | x | ( | 200 | + | 10 | x | ) |
| = | 16000 | + | 800 | x | − | x | ( | 200 | + | 10 | x | ) |
| = | 16000 | + | 800 | x | − | x | × | 200 | − | x | × | 10 | x |
| = | 16000 | + | 600 | x | − | x | × | 10 | x |
| 16000 | + | 600 | x | − | x | × | 10 | x | = | 9000 |