# Rule of 72: Doubling Time Estimation The Rule of 72 is a quick way to how many years it will take for an investment to double, given a fixed annual interest rate. ## Step-by-Step Solution ### Step 1: Understand the Rule of 72 Formula \[ \text{Doubling Time (years)} = \frac{72}{text{Interest Rate (\% per year)}} \] ### Step 2: Plug in the Interest Rate Given interest rate: **9% per year** \[ \text{Doubling Time} = \frac{72}{9} \] ### Step 3: Calculate \[ \frac{72}{9} = 8 \] ### Step 4: State the Answer It will take approximately **8 years** for the investment to double at a 9% annual interest rate using the Rule of 72.