| ( | 30 | + | x | ) | ( | 20 | + | x | ) | − | 600 | = | 875 |
| Left side of the equation = | 30 | ( | 20 | + | x | ) | + | x | ( | 20 | + | x | ) | − | 600 |
| = | 30 | × | 20 | + | 30 | x | + | x | ( | 20 | + | x | ) | − | 600 |
| = | 600 | + | 30 | x | + | x | ( | 20 | + | x | ) | − | 600 |
| = | 0 | + | 30 | x | + | x | ( | 20 | + | x | ) |
| = | 0 | + | 30 | x | + | x | × | 20 | + | x | x |
| = | 0 | + | 50 | x | + | x | x |
| 0 | + | 50 | x | + | x | x | = | 875 |