| ( | x | − | 20 | ) | ( | 1000 | − | 20 | x | ) | = | 4320 |
| Left side of the equation = | x | ( | 1000 | − | 20 | x | ) | − | 20 | ( | 1000 | − | 20 | x | ) |
| = | x | × | 1000 | − | x | × | 20 | x | − | 20 | ( | 1000 | − | 20 | x | ) |
| = | 1000 | x | − | x | × | 20 | x | − | 20 | × | 1000 | + | 20 | × | 20 | x |
| = | 1000 | x | − | x | × | 20 | x | − | 20000 | + | 400 | x |
| = | 1400 | x | − | x | × | 20 | x | − | 20000 |
| 1400 | x | − | x | × | 20 | x | − | 20000 | = | 4320 |