| 180 | − | 30 | − | ( | 240 | − | y | ) | = | 40 | − | ( | 180 | − | 50 | − | y | ) |
| Left side of the equation = | 150 | − | ( | 240 | − | y | ) |
| 150 | − | ( | 240 | − | y | ) | = | 40 | − | ( | 180 | − | 50 | − | y | ) |
| Left side of the equation = | 150 | − | 240 | + | y |
| = | - | 90 | + | y |
| - | 90 | + | y | = | 40 | − | ( | 180 | − | 50 | − | y | ) |
| Right side of the equation = | 40 | − | 180 | + | 50 | + | y |
| = | - | 90 | + | y |
| - | 90 | + | y | = | - | 90 | + | y |
| y | − | y | = | - | 90 | + | 90 |
| 0 | y | = | - | 90 | + | 90 |
| 0 | y | = | - | 0 |