Interior Angles of Regular Polygons

Let polygon A have $n$ sides and polygon B have $2n$ sides. Interior angle of polygon A = $\frac{(n-2) \times 180^\circ}{n}$. Interior angle of polygon B = $\frac{(2n-2) \times 180^\circ}{2n}$. Given ratio of interior angles: $\frac{\frac{n-2}{n}}{\frac{2n-2}{2n}} = \frac{3}{4} \implies \frac{2(n-2)}{2n-2} = \frac{3}{4} \implies \frac{n-2}{n-1} = \frac{3}{4}$. $4n - 8 = 3n - 3 \implies n = 5$. Therefore, the number of sides of B is $2n = 10$.