Final answer:
Cassidy's savings will take about 8 years to double.
Step-by-step explanation:
To determine how long it will take for Cassidy's savings to double, we can use the rule of 72. The rule of 72 states that to find the number of years it takes for an investment to double, divide 72 by the annual interest rate. In this case, Cassidy's savings earn 9% interest annually, so we divide 72 by 9 to get 8. Therefore, it will take about 8 years for her savings to double.