| ( | 2 | + | 4 | x | ) | ( | 40 | − | x | ) | = | 1920 |
| Left side of the equation = | 2 | ( | 40 | − | x | ) | + | 4 | x | ( | 40 | − | x | ) |
| = | 2 | × | 40 | − | 2 | x | + | 4 | x | ( | 40 | − | x | ) |
| = | 80 | − | 2 | x | + | 4 | x | ( | 40 | − | x | ) |
| = | 80 | − | 2 | x | + | 4 | x | × | 40 | − | 4 | x | x |
| = | 80 | − | 2 | x | + | 160 | x | − | 4 | x | x |
| = | 80 | + | 158 | x | − | 4 | x | x |
| 80 | + | 158 | x | − | 4 | x | x | = | 1920 |