| ( | 50 | + | x | ) | ( | 8000 | − | 100 | x | ) | = | 42000 |
| Left side of the equation = | 50 | ( | 8000 | − | 100 | x | ) | + | x | ( | 8000 | − | 100 | x | ) |
| = | 50 | × | 8000 | − | 50 | × | 100 | x | + | x | ( | 8000 | − | 100 | x | ) |
| = | 400000 | − | 5000 | x | + | x | ( | 8000 | − | 100 | x | ) |
| = | 400000 | − | 5000 | x | + | x | × | 8000 | − | x | × | 100 | x |
| = | 400000 | + | 3000 | x | − | x | × | 100 | x |
| 400000 | + | 3000 | x | − | x | × | 100 | x | = | 42000 |