| 312 | = | 80 | a | × | 1 | ÷ | ( | 80 | + | 405 | ) |
| Multiply both sides of the equation by: | ( | 80 | + | 405 | ) |
| 312 | ( | 80 | + | 405 | ) | = | 80 | a | × | 1 |
| 312 | × | 80 | + | 312 | × | 405 | = | 80 | a | × | 1 |
| 24960 | + | 126360 | = | 80 | a |
| 151320 | = | 80 | a |
| - | 80 | a | = | - | 151320 |
| 151320 | = | 80 | a |
| 80 | a | = | 151320 |
| a | = | 151320 | ÷ | 80 |
| = | 151320 | × | 1 80 |
| = | 3783 | × | 1 2 |
| a | = | 3783 2 |
| a | = | 1891.5 |