Two single-core power cables have total conductor resistances of $0.7\;\Omega$ and $0.5\;\Omega$, respectively, and their insulation resistances (between core and sheath) are $600\;M\Omega$ and $900\;M\Omega$, respectively. When the two cables are joined in series, the ratio of insulation resistance to conductor resistance is ___________$\times 10^{6}$.