| ( | 80 | − | x | ) | ( | 60 | + | 5 | x | ) | = | 7200 |
| Left side of the equation = | 80 | ( | 60 | + | 5 | x | ) | − | x | ( | 60 | + | 5 | x | ) |
| = | 80 | × | 60 | + | 80 | × | 5 | x | − | x | ( | 60 | + | 5 | x | ) |
| = | 4800 | + | 400 | x | − | x | ( | 60 | + | 5 | x | ) |
| = | 4800 | + | 400 | x | − | x | × | 60 | − | x | × | 5 | x |
| = | 4800 | + | 340 | x | − | x | × | 5 | x |
| 4800 | + | 340 | x | − | x | × | 5 | x | = | 7200 |