How many sides does a regular polygon have whose interior and exterior angle are in ratio 2 : 1 ?

Each exterior angle of n sided polygon is
$${\text{ = }}\left( {\frac{{360}}{n}} \right)$$
And each interior angle of n sided polygon
$$\eqalign{ & {\text{ = }}\frac{{\left( {n - 2} \right) \times 180}}{n} \cr & \therefore \frac{{\frac{{\left( {n - 2} \right) \times 180}}{n}}}{{\frac{{360}}{n}}} = \frac{2}{1} \cr & \Rightarrow \frac{{\left( {n - 2} \right)}}{2} = 2 \cr & \Rightarrow n - 2 = 4 \cr & \Rightarrow n = 6 \cr} $$