What annual growth rate is needed for a country to double its output in 7 years? In 35 years? In 70 years? In 140 years?

Economists use the concept “rule of 70” to evaluate the approximate number of years an economy will take to double its size. The mathematical expression under the rule of 70 is given below:

$ApproximatenumberofyearsrequiredtodoubletherealGDP=\frac{70}{Annualpercentagerateofgrowth}$

The required annual growth rate for a country to double its output can be calculated by using the “rule of 70” formula.

• The required annual percentage growth for a country to double its output in 7 years is:

$$\begin{gathered} Annualpercentagerateofgrowth=\frac{70}{7} \\ =10percent \end{gathered}$$

The required annual growth rate is 10 percent.

• The required annual percentage growth for a country to double its output in 7 years is:

$$\begin{gathered} Annualpercentagerateofgrowth=\frac{70}{35} \\ =2percent \end{gathered}$$

The required annual growth rate is 2 percent.

• The required annual percentage growth for a country to double its output in 7 years is:

$$\begin{gathered} Annualpercentagerateofgrowth=\frac{70}{70} \\ =1percent \end{gathered}$$

The required annual growth rate is 1 percent

• The required annual percentage growth for a country to double its output in 7 years is:

$$\begin{gathered} Annualperce\tan gerateofgrowth=\frac{70}{140} \\ =0.5percent \end{gathered}$$

The required annual growth rate is 0.5 percent.