| ( | 60 | − | x | ) | ( | 120 | + | 10 | x | ) | = | 1800 |
| Left side of the equation = | 60 | ( | 120 | + | 10 | x | ) | − | x | ( | 120 | + | 10 | x | ) |
| = | 60 | × | 120 | + | 60 | × | 10 | x | − | x | ( | 120 | + | 10 | x | ) |
| = | 7200 | + | 600 | x | − | x | ( | 120 | + | 10 | x | ) |
| = | 7200 | + | 600 | x | − | x | × | 120 | − | x | × | 10 | x |
| = | 7200 | + | 480 | x | − | x | × | 10 | x |
| 7200 | + | 480 | x | − | x | × | 10 | x | = | 1800 |