| ( | 15 | − | a | ) | ( | 400 | + | 5 | a | ) | = | 4250 |
| Left side of the equation = | 15 | ( | 400 | + | 5 | a | ) | − | a | ( | 400 | + | 5 | a | ) |
| = | 15 | × | 400 | + | 15 | × | 5 | a | − | a | ( | 400 | + | 5 | a | ) |
| = | 6000 | + | 75 | a | − | a | ( | 400 | + | 5 | a | ) |
| = | 6000 | + | 75 | a | − | a | × | 400 | − | a | × | 5 | a |
| = | 6000 | − | 325 | a | − | a | × | 5 | a |
| 6000 | − | 325 | a | − | a | × | 5 | a | = | 4250 |