| ( | X | − | 20 | ) | ( | - | 20 | + | 80 | ) | = | 150 |
| Left side of the equation = | X | ( | - | 20 | + | 80 | ) | − | 20 | ( | - | 20 | + | 80 | ) |
| = | - | X | × | 20 | + | X | × | 80 | − | 20 | ( | - | 20 | + | 80 | ) |
| = | 60 | X | − | 20 | ( | - | 20 | + | 80 | ) |
| = | 60 | X | + | 20 | × | 20 | − | 20 | × | 80 |
| = | 60 | X | + | 400 | − | 1600 |
| = | 60 | X | − | 1200 |
| 60 | X | − | 1200 | = | 150 |
| 60 | X | = | 150 | + | 1200 |
| 60 | X | = | 1350 |
| X | = | 1350 | ÷ | 60 |
| = | 1350 | × | 1 60 |
| = | 45 | × | 1 2 |
| X | = | 45 2 |
| X | = | 22.5 |