| ( | 15 | + | x | ) | ( | 500 | − | 20 | x | ) | = | 8000 |
| Left side of the equation = | 15 | ( | 500 | − | 20 | x | ) | + | x | ( | 500 | − | 20 | x | ) |
| = | 15 | × | 500 | − | 15 | × | 20 | x | + | x | ( | 500 | − | 20 | x | ) |
| = | 7500 | − | 300 | x | + | x | ( | 500 | − | 20 | x | ) |
| = | 7500 | − | 300 | x | + | x | × | 500 | − | x | × | 20 | x |
| = | 7500 | + | 200 | x | − | x | × | 20 | x |
| 7500 | + | 200 | x | − | x | × | 20 | x | = | 8000 |