| ( | 50 | + | x | ) | ( | 800 | − | 20 | x | ) | = | 12000 |
| Left side of the equation = | 50 | ( | 800 | − | 20 | x | ) | + | x | ( | 800 | − | 20 | x | ) |
| = | 50 | × | 800 | − | 50 | × | 20 | x | + | x | ( | 800 | − | 20 | x | ) |
| = | 40000 | − | 1000 | x | + | x | ( | 800 | − | 20 | x | ) |
| = | 40000 | − | 1000 | x | + | x | × | 800 | − | x | × | 20 | x |
| = | 40000 | − | 200 | x | − | x | × | 20 | x |
| 40000 | − | 200 | x | − | x | × | 20 | x | = | 12000 |