Pressures at the bottom of the tank must be equal to the pressures due to water of height $h.$

If $d_w \mathrm{a}\mathrm{n}\mathrm{d}\mathrm{ }{\mathrm{d}}_0$ be the densities of water and oil respectively, Pressure at the bottom of the tank

$=h_w{d}_{w}g+h_0{d}_{0}g$

If this pressure be equivalent to pressure due to water of height $h.$

Then, $h{d}_{w}g=h_w{d}_{w}g+h_0{d}_{0}g$

$h=h_w+\frac{h_0{d}_0}{d_w}=100+\frac{400\times 0.9}{1}$

$=100+360=460$

According to Torricelli's theorem

$v=\sqrt{2gh}=\sqrt{2\times 980\times 460}$

$=\sqrt{920\times 980\mathrm{ }} \mathrm{cm} {\mathrm{s}}^{-1}$