If T is the surface tension of soap solution, the amount of work done in blowing a soap bubble from a diameter D to 2D ??

The work done in making a bubble by blowing is stored in the form of energy in the surface of the bubble.Soap bubble has two surfaces,external and internal.Area of both the surfaces is 

$A=2\times 4{\mathrm{\pi r}}^2 \mathrm{where} \mathrm{r} \mathrm{is} \mathrm{radius} \mathrm{of} \mathrm{bubble}$

$$\begin{gathered} A=2\times 4{\mathrm{\pi r}}^2 \\ \mathrm{work} \mathrm{done} =\mathrm{surface} \mathrm{tension} \times \mathrm{area} \\ {\mathrm{W}}_1=\mathrm{T}8{{\mathrm{\pi r}}_1}^2 \\ \mathrm{Now} \mathrm{diametre} \mathrm{doubled} {\mathrm{D}}_1=\raisebox{1ex}{${\mathrm{r}}_1$}\!\left/ \!\raisebox{-1ex}{$2$}\right. \\ {\mathrm{D}}_2=2{\mathrm{D}}_1 \\ \frac{{\mathrm{r}}_2}{2}=\frac{2{\mathrm{r}}_1}{2} {\mathrm{r}}_2=2{\mathrm{r}}_1 \\ {\mathrm{W}}_2=8{{\mathrm{T\pi r}}_2}^2=8\mathrm{T\pi }{\left(2{\mathrm{r}}_1\right)}^2=32{{\mathrm{T\pi r}}_1}^2 \\ \boxed{{\mathrm{W}}_2=4{\mathrm{W}}_1} \end{gathered}$$