| 15000 | a | + | ( | 240 | − | 15 | a | ) | ÷ | 12 | = | 240000 |
| Left side of the equation = | 15000 | a | + | 240 | × | 1 12 | − | 15 | a | × | 1 12 |
| = | 15000 | a | + | 20 | − | 5 4 | a |
| = | 59995 4 | a | + | 20 |
59995 4 | a | + | 20 | = | 240000 |
59995 4 | a | = | 240000 | − | 20 |
59995 4 | a | = | 239980 |
| a | = | 239980 | ÷ | 59995 4 |
| = | 239980 | × | 4 59995 |
| = | 4 | × | 4 |
| a | = | 16 |