we know that
volume of a sphere=(4/3)*pi*r³
let
A-----> smaller sphere
B-----> larger sphere
VA=(4/3)*pi*rA³
VB=(4/3)*pi*rB³
VA/VB=1/8--------> (1/8)=[(4/3)*pi*rA³]/[(4/3)*pi*rB³]
(1/8)=[rA³]/[rB³]-----> (1/8)=[rA/rB]³-----> rA/rB=∛(1/8)----> rA/rB=1/2
the answer is
the ratio of their radii is (1/2)