| ( | 30 | − | x | ) | ( | 20 | + | 5 | x | ) | − | 20 | x | = | 240 |
| Left side of the equation = | 30 | ( | 20 | + | 5 | x | ) | − | x | ( | 20 | + | 5 | x | ) | − | 20 | x |
| = | 30 | × | 20 | + | 30 | × | 5 | x | − | x | ( | 20 | + | 5 | x | ) | − | 20 | x |
| = | 600 | + | 150 | x | − | x | ( | 20 | + | 5 | x | ) | − | 20 | x |
| = | 600 | + | 130 | x | − | x | ( | 20 | + | 5 | x | ) |
| = | 600 | + | 130 | x | − | x | × | 20 | − | x | × | 5 | x |
| = | 600 | + | 110 | x | − | x | × | 5 | x |
| 600 | + | 110 | x | − | x | × | 5 | x | = | 240 |