| ( | 40 | − | 4 | x | ) | ( | 25 | − | x | ) | = | 600 |
| Left side of the equation = | 40 | ( | 25 | − | x | ) | − | 4 | x | ( | 25 | − | x | ) |
| = | 40 | × | 25 | − | 40 | x | − | 4 | x | ( | 25 | − | x | ) |
| = | 1000 | − | 40 | x | − | 4 | x | ( | 25 | − | x | ) |
| = | 1000 | − | 40 | x | − | 4 | x | × | 25 | + | 4 | x | x |
| = | 1000 | − | 40 | x | − | 100 | x | + | 4 | x | x |
| = | 1000 | − | 140 | x | + | 4 | x | x |
| 1000 | − | 140 | x | + | 4 | x | x | = | 600 |