| ( | 200 | + | 20 | x | ) | ( | 50 | − | x | ) | = | 11520 |
| Left side of the equation = | 200 | ( | 50 | − | x | ) | + | 20 | x | ( | 50 | − | x | ) |
| = | 200 | × | 50 | − | 200 | x | + | 20 | x | ( | 50 | − | x | ) |
| = | 10000 | − | 200 | x | + | 20 | x | ( | 50 | − | x | ) |
| = | 10000 | − | 200 | x | + | 20 | x | × | 50 | − | 20 | x | x |
| = | 10000 | − | 200 | x | + | 1000 | x | − | 20 | x | x |
| = | 10000 | + | 800 | x | − | 20 | x | x |
| 10000 | + | 800 | x | − | 20 | x | x | = | 11520 |