| 1215 | = | - | 15 | × | 25 | × | 25 | + | ( | 600 | + | 15 | a | ) | × | 25 | − | 450 | a | − | 4500 |
| Right side of the equation = | - | 9375 | + | ( | 600 | + | 15 | a | ) | × | 25 | − | 450 | a | − | 4500 |
| = | - | 13875 | + | ( | 600 | + | 15 | a | ) | × | 25 | − | 450 | a |
| 1215 | = | - | 13875 | + | ( | 600 | + | 15 | a | ) | × | 25 | − | 450 | a |
| Right side of the equation = | - | 13875 | + | 600 | × | 25 | + | 15 | a | × | 25 | − | 450 | a |
| = | - | 13875 | + | 15000 | + | 375 | a | − | 450 | a |
| = | 1125 | − | 75 | a |
| 1215 | = | 1125 | − | 75 | a |
| 75 | a | = | 1125 | − | 1215 |
| 75 | a | = | - | 90 |
| a | = | - | 90 | ÷ | 75 |
| = | - | 90 | × | 1 75 |
| = | - | 6 | × | 1 5 |
| a | = | - | 6 5 |
| a | = | - 1.2 |