Consider a cuboid of sides $2 x , 4 x$ and $5 x$ and a closed hemisphere of radius $r$. If the sum of their surface areas is constant $k$, then the ratio $x : r$, for which the sum of their volumes is maximum, is\\
(1) $2 : 5$\\
(2) $19 : 45$\\
(3) $3 : 8$\\
(4) $19 : 15$