| ( | 15 | − | a | ) | ( | 500 | + | 5 | a | ) | = | 4250 |
| Left side of the equation = | 15 | ( | 500 | + | 5 | a | ) | − | a | ( | 500 | + | 5 | a | ) |
| = | 15 | × | 500 | + | 15 | × | 5 | a | − | a | ( | 500 | + | 5 | a | ) |
| = | 7500 | + | 75 | a | − | a | ( | 500 | + | 5 | a | ) |
| = | 7500 | + | 75 | a | − | a | × | 500 | − | a | × | 5 | a |
| = | 7500 | − | 425 | a | − | a | × | 5 | a |
| 7500 | − | 425 | a | − | a | × | 5 | a | = | 4250 |