To calculate the doubling time with a given growth rate, we use the Rule of 70. The formula for the Rule of 70 is:
[tex]\[
\text{Doubling Time} = \frac{70}{r}
\][/tex]
where [tex]\( r \)[/tex] is the growth rate in percentage.
In this problem, the growth rate [tex]\( r \)[/tex] is given as 4%. Plug this value into the formula:
[tex]\[
\text{Doubling Time} = \frac{70}{4}
\][/tex]
Now, perform the division:
[tex]\[
\frac{70}{4} = 17.5
\][/tex]
Thus, the doubling time, when the growth rate is 4%, is [tex]\( 17.5 \)[/tex] years, making the correct answer:
[tex]\[
\boxed{17.5 \text{ years}}
\][/tex]
