| ( | 20 | − | 2 | x | ) | ( | 15 | − | x | ) | = | 100 |
| Left side of the equation = | 20 | ( | 15 | − | x | ) | − | 2 | x | ( | 15 | − | x | ) |
| = | 20 | × | 15 | − | 20 | x | − | 2 | x | ( | 15 | − | x | ) |
| = | 300 | − | 20 | x | − | 2 | x | ( | 15 | − | x | ) |
| = | 300 | − | 20 | x | − | 2 | x | × | 15 | + | 2 | x | x |
| = | 300 | − | 20 | x | − | 30 | x | + | 2 | x | x |
| = | 300 | − | 50 | x | + | 2 | x | x |
| 300 | − | 50 | x | + | 2 | x | x | = | 100 |