Final answer:
The Rule of 70 indicates that the number of years it takes for a variable to double is about 70 divided by the annual percentage growth rate. The correct response is A. double; 70 divided by the annual percentage growth rate of the variable.
Step-by-step explanation:
The Rule of 70 states that the number of years it takes for the level of any variable to double is approximately 70 divided by the annual percentage growth rate of the variable. Therefore, the correct answer to the student's question is A. double; 70 divided by the annual percentage growth rate of the variable. This rule is particularly useful when dealing with exponential growth rates and compounding, such as in the contexts of population growth, financial investments, or economic development. For instance, a town with a steady annual growth rate of 7% would see its size double every 10 years according to the Rule of 70. Similarly, if a country's GDP grows at an annual rate of 3%, it would take approximately 23.33 years (70/3) for the GDP to double. The rule of 70 provides a quick, albeit approximate, way to estimate doubling times for growth processes.