$q = Q _{0} e ^{\frac{ T _{1}}{ RC }}=\frac{ Q _{0}}{\sqrt{2}}$
and $q = Q _{0} e ^{\frac{ t _{1}}{ RC }}=\frac{ Q _{0}}{8}=\left(\frac{ Q _{0}}{\sqrt{2}}\right)^{6}$
$t _{2}=6 t _{1}$
$\frac{ t _{1}}{ t _{2}}=\frac{1}{6}$


Reason : The electric field between the plates of a charged isolated capacitance increases when dielectric fills whole space between plates.