| ( | x | − | 20 | ) | ( | 480 | − | 10 | x | ) | = | 1920 |
| Left side of the equation = | x | ( | 480 | − | 10 | x | ) | − | 20 | ( | 480 | − | 10 | x | ) |
| = | x | × | 480 | − | x | × | 10 | x | − | 20 | ( | 480 | − | 10 | x | ) |
| = | 480 | x | − | x | × | 10 | x | − | 20 | × | 480 | + | 20 | × | 10 | x |
| = | 480 | x | − | x | × | 10 | x | − | 9600 | + | 200 | x |
| = | 680 | x | − | x | × | 10 | x | − | 9600 |
| 680 | x | − | x | × | 10 | x | − | 9600 | = | 1920 |