| 10000 | = | 20000 | ÷ | ( | 1 | + | r | ) | × | 20 |
| Multiply both sides of the equation by: | ( | 1 | + | r | ) |
| 10000 | ( | 1 | + | r | ) | = | 20000 | × | 20 |
| 10000 | × | 1 | + | 10000 | r | = | 20000 | × | 20 |
| 10000 | + | 10000 | r | = | 400000 |
| 10000 | r | = | 400000 | − | 10000 |
| 10000 | r | = | 390000 |
| r | = | 390000 | ÷ | 10000 |
| = | 390000 | × | 1 10000 |
| = | 39 | × | 1 |
| r | = | 39 |