| ( | 30 | − | x | ) | ( | 100 | + | 20 | x | ) | = | 1120 |
| Left side of the equation = | 30 | ( | 100 | + | 20 | x | ) | − | x | ( | 100 | + | 20 | x | ) |
| = | 30 | × | 100 | + | 30 | × | 20 | x | − | x | ( | 100 | + | 20 | x | ) |
| = | 3000 | + | 600 | x | − | x | ( | 100 | + | 20 | x | ) |
| = | 3000 | + | 600 | x | − | x | × | 100 | − | x | × | 20 | x |
| = | 3000 | + | 500 | x | − | x | × | 20 | x |
| 3000 | + | 500 | x | − | x | × | 20 | x | = | 1120 |