| ( | a | − | 20 | ) | ( | 800 | − | 10 | a | ) | = | 8000 |
| Left side of the equation = | a | ( | 800 | − | 10 | a | ) | − | 20 | ( | 800 | − | 10 | a | ) |
| = | a | × | 800 | − | a | × | 10 | a | − | 20 | ( | 800 | − | 10 | a | ) |
| = | 800 | a | − | a | × | 10 | a | − | 20 | × | 800 | + | 20 | × | 10 | a |
| = | 800 | a | − | a | × | 10 | a | − | 16000 | + | 200 | a |
| = | 1000 | a | − | a | × | 10 | a | − | 16000 |
| 1000 | a | − | a | × | 10 | a | − | 16000 | = | 8000 |