| ( | a | − | 20 | ) | ( | - | 5 | a | + | 200 | ) | = | 500 |
| Left side of the equation = | a | ( | - | 5 | a | + | 200 | ) | − | 20 | ( | - | 5 | a | + | 200 | ) |
| = | - | a | × | 5 | a | + | a | × | 200 | − | 20 | ( | - | 5 | a | + | 200 | ) |
| = | - | a | × | 5 | a | + | 200 | a | + | 20 | × | 5 | a | − | 20 | × | 200 |
| = | - | a | × | 5 | a | + | 200 | a | + | 100 | a | − | 4000 |
| = | - | a | × | 5 | a | + | 300 | a | − | 4000 |
| - | a | × | 5 | a | + | 300 | a | − | 4000 | = | 500 |