| ( | 500 | − | 100 | x | ) | ( | 1 | + | x | ) | = | 800 |
| Left side of the equation = | 500 | ( | 1 | + | x | ) | − | 100 | x | ( | 1 | + | x | ) |
| = | 500 | × | 1 | + | 500 | x | − | 100 | x | ( | 1 | + | x | ) |
| = | 500 | + | 500 | x | − | 100 | x | ( | 1 | + | x | ) |
| = | 500 | + | 500 | x | − | 100 | x | × | 1 | − | 100 | x | x |
| = | 500 | + | 500 | x | − | 100 | x | − | 100 | x | x |
| = | 500 | + | 400 | x | − | 100 | x | x |
| 500 | + | 400 | x | − | 100 | x | x | = | 800 |