| ( | 180 | − | 50 | − | x | ) | + | ( | 180 | − | 30 | − | ( | 40 | − | x | ) | ) | = | 240 |
| Left side of the equation = | 180 | − | 50 | − | x | + | ( | 180 | − | 30 | − | ( | 40 | − | x | ) | ) |
| = | 130 | − | x | + | ( | 180 | − | 30 | − | ( | 40 | − | x | ) | ) |
| = | 130 | − | x | + | 180 | − | 30 | − | ( | 40 | − | x | ) |
| = | 280 | − | x | − | ( | 40 | − | x | ) |
| = | 280 | − | x | − | 40 | + | x |
| = | 240 | − | 0 | x |
| 240 | − | 0 | x | = | 240 |
| = | 240 | − | 240 |