| 1200 | − | ( | 40 | − | x | ) | ( | 30 | − | x | ) | = | 325 |
| Left side of the equation = | 1200 | − | 40 | ( | 30 | − | x | ) | + | x | ( | 30 | − | x | ) |
| = | 1200 | − | 40 | × | 30 | + | 40 | x | + | x | ( | 30 | − | x | ) |
| = | 1200 | − | 1200 | + | 40 | x | + | x | ( | 30 | − | x | ) |
| = | 0 | + | 40 | x | + | x | ( | 30 | − | x | ) |
| = | 0 | + | 40 | x | + | x | × | 30 | − | x | x |
| = | 0 | + | 70 | x | − | x | x |
| 0 | + | 70 | x | − | x | x | = | 325 |