| ( | 20 | − | x | ) | ( | 240 | + | 20 | x | ) | = | 6500 |
| Left side of the equation = | 20 | ( | 240 | + | 20 | x | ) | − | x | ( | 240 | + | 20 | x | ) |
| = | 20 | × | 240 | + | 20 | × | 20 | x | − | x | ( | 240 | + | 20 | x | ) |
| = | 4800 | + | 400 | x | − | x | ( | 240 | + | 20 | x | ) |
| = | 4800 | + | 400 | x | − | x | × | 240 | − | x | × | 20 | x |
| = | 4800 | + | 160 | x | − | x | × | 20 | x |
| 4800 | + | 160 | x | − | x | × | 20 | x | = | 6500 |