I'm not sure if I understand the question, but will take a crack at it:
Let the radius of the hemisphere = a, then:
The radius of the balloon =a^(1/3)
So the ratio of the 2 radii =a : a^(1/3)
Volume =4/3*pi*r^3
[4/3*pi*(a^(1/3))^3] =[4/3*pi*a^3]/2, solve for a
a =+or- sqrt(2)
P.S. Somebody should look at this. Thanks.
Let V be the volume of the balloon.
Let r1 be the radius of the sphere.
Let r2 be the radius of the hemisphere.
\(V=\frac43\pi (r_{\small1})^3\\~\\ \frac3{4\pi}\cdot V=\frac3{4\pi}\cdot\frac43\pi (r_{\small1})^3\\~\\ \frac{3V}{4\pi}=(r_{\small1})^3\\~\\ \sqrt[3]{\frac{3V}{4\pi}}=\sqrt[3]{(r_{\small1})^3}\\~\\ \sqrt[3]{\frac{3V}{4\pi}}=r_{\small1}\) Solve this equation for r1
\(V=\frac12\cdot\frac43\pi (r_{\small2})^3\\~\\ V=\frac23\pi (r_{\small2})^3\\~\\ \frac3{2\pi}\cdot V=\frac3{2\pi}\cdot\frac23\pi (r_{\small2})^3\\~\\ \frac{3V}{2\pi}=(r_{\small2})^3\\~\\ \sqrt[3]{\frac{3V}{2\pi}}=\sqrt[3]{(r_{\small2})^3}\\~\\ \sqrt[3]{\frac{3V}{2\pi}}=r_{\small2} \) Solve this equation for r2 .
The ratio of r1 to r2 can be expressed in the form \(\sqrt[3]{a}\) for some real number a .
\(\dfrac{r_{\small1}}{r_{\small2}}=\sqrt[3]{a} \\~\\ \dfrac{\sqrt[3]{\frac{3V}{4\pi}}}{\sqrt[3]{\frac{3V}{2\pi}}}=\sqrt[3]{a}\\~\\ \dfrac{\frac{3V}{4\pi}}{\frac{3V}{2\pi}}=a\\~\\ \frac{3V}{4\pi}\cdot\frac{2\pi}{3V}=a\\~\\ \frac14\cdot\frac21=a\\~\\ \frac24=a \\~\\ \frac12=a\) Plug in the equivalent expressions of r1 and r2 and solve for a .