| ( | a | − | 20 | ) | ( | 80 | − | 2 | a | ) | = | 150 |
| Left side of the equation = | a | ( | 80 | − | 2 | a | ) | − | 20 | ( | 80 | − | 2 | a | ) |
| = | a | × | 80 | − | a | × | 2 | a | − | 20 | ( | 80 | − | 2 | a | ) |
| = | 80 | a | − | a | × | 2 | a | − | 20 | × | 80 | + | 20 | × | 2 | a |
| = | 80 | a | − | a | × | 2 | a | − | 1600 | + | 40 | a |
| = | 120 | a | − | a | × | 2 | a | − | 1600 |
| 120 | a | − | a | × | 2 | a | − | 1600 | = | 150 |