| ( | 20 | − | a | ) | ( | 20 | − | a | ) | − | a | a | = | 40 |
| Left side of the equation = | 20 | ( | 20 | − | a | ) | − | a | ( | 20 | − | a | ) | − | a | a |
| = | 20 | × | 20 | − | 20 | a | − | a | ( | 20 | − | a | ) | − | a | a |
| = | 400 | − | 20 | a | − | a | ( | 20 | − | a | ) | − | a | a |
| = | 400 | − | 20 | a | − | a | × | 20 | + | a | a | − | a | a |
| = | 400 | − | 40 | a | + | a | a | − | a | a |
| 400 | − | 40 | a | + | a | a | − | a | a | = | 40 |