To determine how many years it will take for a country's GDP to double when it grows at a rate of 3.5% per year, we can use the Rule of 70. The Rule of 70 is a simple way to estimate the number of years required for a quantity to double given a fixed annual growth rate.
The formula for the Rule of 70 is:
[tex]\[ \text{Doubling Time} = \frac{70}{\text{Growth Rate}} \][/tex]
Here, the growth rate is given as 3.5% per year. Plugging this into the formula, we get:
[tex]\[ \text{Doubling Time} = \frac{70}{3.5} \][/tex]
When we calculate this, we find:
[tex]\[ \text{Doubling Time} = 20 \][/tex]
Therefore, it will take approximately 20 years for the GDP to double at an annual growth rate of 3.5%.
So, the correct answer is:
3. About 20 years
