| ( | 60 | + | x | ) | ( | 300 | − | 10 | x | ) | = | 4000 |
| Left side of the equation = | 60 | ( | 300 | − | 10 | x | ) | + | x | ( | 300 | − | 10 | x | ) |
| = | 60 | × | 300 | − | 60 | × | 10 | x | + | x | ( | 300 | − | 10 | x | ) |
| = | 18000 | − | 600 | x | + | x | ( | 300 | − | 10 | x | ) |
| = | 18000 | − | 600 | x | + | x | × | 300 | − | x | × | 10 | x |
| = | 18000 | − | 300 | x | − | x | × | 10 | x |
| 18000 | − | 300 | x | − | x | × | 10 | x | = | 4000 |