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