| ( | 200 | − | x | ) | ( | 80 | − | x | ) | = | 280 | × | 75 |
| Left side of the equation = | 200 | ( | 80 | − | x | ) | − | x | ( | 80 | − | x | ) |
| = | 200 | × | 80 | − | 200 | x | − | x | ( | 80 | − | x | ) |
| = | 16000 | − | 200 | x | − | x | ( | 80 | − | x | ) |
| = | 16000 | − | 200 | x | − | x | × | 80 | + | x | x |
| = | 16000 | − | 280 | x | + | x | x |
| 16000 | − | 280 | x | + | x | x | = | 280 | × | 75 |
| Right side of the equation = | 21000 |
| 16000 | − | 280 | x | + | x | x | = | 21000 |