Q: A sphere of solid material of specific gravity 8 has a concentric spherical cavity and just sinks in water. The ratio of radius of cavity to that of outer radius of the sphere must be
(a) $ \displaystyle \frac{7^{1/3}}{2} $
(b) $ \displaystyle \frac{5^{1/3}}{2} $
(c) $ \displaystyle \frac{9^{1/3}}{2} $
(d) $ \displaystyle \frac{3^{1/3}}{2} $
Ans: (a)