| ( | x | − | 80 | ) | ( | 100 | − | x | ) | × | 5 | + | 50 | = | 1080 |
| Left side of the equation = | x | ( | 100 | − | x | ) | × | 5 | − | 80 | ( | 100 | − | x | ) | × | 5 | + | 50 |
| = | x | ( | 100 | − | x | ) | × | 5 | − | 400 | ( | 100 | − | x | ) | + | 50 |
| = | x | × | 100 | × | 5 | − | x | x | × | 5 | − | 400 | ( | 100 | − | x | ) | + | 50 |
| = | x | × | 500 | − | x | x | × | 5 | − | 400 | ( | 100 | − | x | ) | + | 50 |
| = | 500 | x | − | x | x | × | 5 | − | 400 | × | 100 | + | 400 | x | + | 50 |
| = | 500 | x | − | x | x | × | 5 | − | 40000 | + | 400 | x | + | 50 |
| = | 900 | x | − | x | x | × | 5 | − | 39950 |
| 900 | x | − | x | x | × | 5 | − | 39950 | = | 1080 |