| ( | 20 | − | x | ) | ( | 500 | + | 50 | x | ) | = | 10000 |
| Left side of the equation = | 20 | ( | 500 | + | 50 | x | ) | − | x | ( | 500 | + | 50 | x | ) |
| = | 20 | × | 500 | + | 20 | × | 50 | x | − | x | ( | 500 | + | 50 | x | ) |
| = | 10000 | + | 1000 | x | − | x | ( | 500 | + | 50 | x | ) |
| = | 10000 | + | 1000 | x | − | x | × | 500 | − | x | × | 50 | x |
| = | 10000 | + | 500 | x | − | x | × | 50 | x |
| 10000 | + | 500 | x | − | x | × | 50 | x | = | 10000 |