| 120 | ( | 1 | − | x | ) | ( | 1 | − | x | ) | = | 80 |
| Left side of the equation = | 120 | × | 1 | ( | 1 | − | x | ) | − | 120 | x | ( | 1 | − | x | ) |
| = | 120 | ( | 1 | − | x | ) | − | 120 | x | ( | 1 | − | x | ) |
| = | 120 | × | 1 | − | 120 | x | − | 120 | x | ( | 1 | − | x | ) |
| = | 120 | − | 120 | x | − | 120 | x | ( | 1 | − | x | ) |
| = | 120 | − | 120 | x | − | 120 | x | × | 1 | + | 120 | x | x |
| = | 120 | − | 120 | x | − | 120 | x | + | 120 | x | x |
| = | 120 | − | 240 | x | + | 120 | x | x |
| 120 | − | 240 | x | + | 120 | x | x | = | 80 |