| ( | 20 | + | 2 | x | ) | ( | 800 | − | x | ) | = | 1200 |
| Left side of the equation = | 20 | ( | 800 | − | x | ) | + | 2 | x | ( | 800 | − | x | ) |
| = | 20 | × | 800 | − | 20 | x | + | 2 | x | ( | 800 | − | x | ) |
| = | 16000 | − | 20 | x | + | 2 | x | ( | 800 | − | x | ) |
| = | 16000 | − | 20 | x | + | 2 | x | × | 800 | − | 2 | x | x |
| = | 16000 | − | 20 | x | + | 1600 | x | − | 2 | x | x |
| = | 16000 | + | 1580 | x | − | 2 | x | x |
| 16000 | + | 1580 | x | − | 2 | x | x | = | 1200 |