| 1000 | = | 1200 | × | 5 100 | ÷ | ( | 1 | + | I | ) | + | 1200 | ÷ | ( | 1 | + | I | ) |
| Multiply both sides of the equation by: | ( | 1 | + | I | ) |
| 1000 | ( | 1 | + | I | ) | = | 1200 | × | 5 100 | + | 1200 |
| 1000 | × | 1 | + | 1000 | I | = | 1200 | × | 5 100 | + | 1200 |
| 1000 | + | 1000 | I | = | 60 | + | 1200 |
| 1000 | + | 1000 | I | = | 1260 |
| 1000 | I | = | 1260 | − | 1000 |
| 1000 | I | = | 260 |
| I | = | 260 | ÷ | 1000 |
| = | 260 | × | 1 1000 |
| = | 13 | × | 1 50 |
| I | = | 13 50 |
| I | = | 0.26 |