| ( | 90 | − | 50 | − | x | ) | ( | 20 | + | 2 | x | ) | = | 1200 |
| Left side of the equation = | 90 | ( | 20 | + | 2 | x | ) | − | 50 | ( | 20 | + | 2 | x | ) | − | x | ( | 20 | + | 2 | x | ) |
| = | 90 | × | 20 | + | 90 | × | 2 | x | − | 50 | ( | 20 | + | 2 | x | ) | − | x | ( | 20 | + | 2 | x | ) |
| = | 1800 | + | 180 | x | − | 50 | ( | 20 | + | 2 | x | ) | − | x | ( | 20 | + | 2 | x | ) |
| = | 1800 | + | 180 | x | − | 50 | × | 20 | − | 50 | × | 2 | x | − | x | ( | 20 | + | 2 | x | ) |
| = | 1800 | + | 180 | x | − | 1000 | − | 100 | x | − | x | ( | 20 | + | 2 | x | ) |
| = | 800 | + | 80 | x | − | x | ( | 20 | + | 2 | x | ) |
| = | 800 | + | 80 | x | − | x | × | 20 | − | x | × | 2 | x |
| = | 800 | + | 60 | x | − | x | × | 2 | x |
| 800 | + | 60 | x | − | x | × | 2 | x | = | 1200 |