| ( | 20 | − | a | ) | ( | 100 | + | 10 | a | ) | = | 2160 |
| Left side of the equation = | 20 | ( | 100 | + | 10 | a | ) | − | a | ( | 100 | + | 10 | a | ) |
| = | 20 | × | 100 | + | 20 | × | 10 | a | − | a | ( | 100 | + | 10 | a | ) |
| = | 2000 | + | 200 | a | − | a | ( | 100 | + | 10 | a | ) |
| = | 2000 | + | 200 | a | − | a | × | 100 | − | a | × | 10 | a |
| = | 2000 | + | 100 | a | − | a | × | 10 | a |
| 2000 | + | 100 | a | − | a | × | 10 | a | = | 2160 |