| ( | 20 | + | 2 | x | ) | ( | 40 | − | x | ) | = | 1200 |
| Left side of the equation = | 20 | ( | 40 | − | x | ) | + | 2 | x | ( | 40 | − | x | ) |
| = | 20 | × | 40 | − | 20 | x | + | 2 | x | ( | 40 | − | x | ) |
| = | 800 | − | 20 | x | + | 2 | x | ( | 40 | − | x | ) |
| = | 800 | − | 20 | x | + | 2 | x | × | 40 | − | 2 | x | x |
| = | 800 | − | 20 | x | + | 80 | x | − | 2 | x | x |
| = | 800 | + | 60 | x | − | 2 | x | x |
| 800 | + | 60 | x | − | 2 | x | x | = | 1200 |