| ( | a | − | 18 | ) | ( | 320 | − | 10 | a | ) | = | 400 |
| Left side of the equation = | a | ( | 320 | − | 10 | a | ) | − | 18 | ( | 320 | − | 10 | a | ) |
| = | a | × | 320 | − | a | × | 10 | a | − | 18 | ( | 320 | − | 10 | a | ) |
| = | 320 | a | − | a | × | 10 | a | − | 18 | × | 320 | + | 18 | × | 10 | a |
| = | 320 | a | − | a | × | 10 | a | − | 5760 | + | 180 | a |
| = | 500 | a | − | a | × | 10 | a | − | 5760 |
| 500 | a | − | a | × | 10 | a | − | 5760 | = | 400 |