| ( | 135 | ) | a | + | 180 | ( | 60 | − | a | ) | = | 9900 |
| Left side of the equation = | 135 | a | + | 180 | ( | 60 | − | a | ) |
| = | 135 | a | + | 180 | × | 60 | − | 180 | a |
| = | 135 | a | + | 10800 | − | 180 | a |
| = | - | 45 | a | + | 10800 |
| - | 45 | a | + | 10800 | = | 9900 |
| - | 45 | a | = | 9900 | − | 10800 |
| - | 45 | a | = | - | 900 |
| 900 | = | 45 | a |
| 45 | a | = | 900 |
| a | = | 900 | ÷ | 45 |
| = | 900 | × | 1 45 |
| = | 20 | × | 1 |
| a | = | 20 |