| ( | 24 | + | 4 | x | ) | ( | 40 | − | x | ) | = | 1920 |
| Left side of the equation = | 24 | ( | 40 | − | x | ) | + | 4 | x | ( | 40 | − | x | ) |
| = | 24 | × | 40 | − | 24 | x | + | 4 | x | ( | 40 | − | x | ) |
| = | 960 | − | 24 | x | + | 4 | x | ( | 40 | − | x | ) |
| = | 960 | − | 24 | x | + | 4 | x | × | 40 | − | 4 | x | x |
| = | 960 | − | 24 | x | + | 160 | x | − | 4 | x | x |
| = | 960 | + | 136 | x | − | 4 | x | x |
| 960 | + | 136 | x | − | 4 | x | x | = | 1920 |