Harder compound interest questions may ask for a missing multiplier or a missing number of years. Use \(\text{final amount} = \text{original amount} \times \text{multiplier}^{\text{time}}\), then rearrange or test powers carefully. A missing multiplier usually involves a root, while a missing number of years can be found by comparing powers.
Common mistake: Do not divide the final amount by the original amount and treat that as the yearly multiplier straight away. If the change happened for more than one year, you must also deal with the power.