To find out how many years it will take for $100 to double at an annual interest rate of 10%, we can use the Rule of 72. This rule provides a simple way to estimate the number of years required to double an investment by dividing 72 by the annual interest rate.
Using the Rule of 72: \[ \text{Years to double} = \frac{72}{\text{interest rate}} \] \[ \text{Years to double} = \frac{72}{10} = 7.2 \]
Based on this calculation, it will take approximately 7 years for $100 to double at an annual interest rate of 10%.
Therefore, the answer is b. 7.
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