| ( | 1000 | − | 20 | x | ) | ( | x | − | 20 | ) | = | 0 |
| Left side of the equation = | 1000 | ( | x | − | 20 | ) | − | 20 | x | ( | x | − | 20 | ) |
| = | 1000 | x | − | 1000 | × | 20 | − | 20 | x | ( | x | − | 20 | ) |
| = | 1000 | x | − | 20000 | − | 20 | x | ( | x | − | 20 | ) |
| = | 1000 | x | − | 20000 | − | 20 | x | x | + | 20 | x | × | 20 |
| = | 1000 | x | − | 20000 | − | 20 | x | x | + | 400 | x |
| = | 1400 | x | − | 20000 | − | 20 | x | x |
| 1400 | x | − | 20000 | − | 20 | x | x | = | 0 |