| ( | 8 | − | x | ) | ( | 240 | − | 20 | x | ) | = | 1980 |
| Left side of the equation = | 8 | ( | 240 | − | 20 | x | ) | − | x | ( | 240 | − | 20 | x | ) |
| = | 8 | × | 240 | − | 8 | × | 20 | x | − | x | ( | 240 | − | 20 | x | ) |
| = | 1920 | − | 160 | x | − | x | ( | 240 | − | 20 | x | ) |
| = | 1920 | − | 160 | x | − | x | × | 240 | + | x | × | 20 | x |
| = | 1920 | − | 400 | x | + | x | × | 20 | x |
| 1920 | − | 400 | x | + | x | × | 20 | x | = | 1980 |