| 40 | x | x | + | ( | 90 | − | 30 | x | ) | ( | 90 | − | 30 | x | ) | = | 2500 |
| Left side of the equation = | 40 | x | x | + | 90 | ( | 90 | − | 30 | x | ) | − | 30 | x | ( | 90 | − | 30 | x | ) |
| = | 40 | x | x | + | 90 | × | 90 | − | 90 | × | 30 | x | − | 30 | x | ( | 90 | − | 30 | x | ) |
| = | 40 | x | x | + | 8100 | − | 2700 | x | − | 30 | x | ( | 90 | − | 30 | x | ) |
| = | 40 | x | x | + | 8100 | − | 2700 | x | − | 30 | x | × | 90 | + | 30 | x | × | 30 |
| = | 40 | x | x | + | 8100 | − | 2700 | x | − | 2700 | x | + | 900 | x | x |
| = | 40 | x | x | + | 8100 | − | 5400 | x | + | 900 | x | x |
| 40 | x | x | + | 8100 | − | 5400 | x | + | 900 | x | x | = | 2500 |