A vessel of depth d is half filled with oil of refractive index n _...

The apparent depth formula is given by $n = \dfrac{Realdepth}{Apparentdepth}$.

The apparent depth in this case is given by

$d ' = \dfrac{{d}_{1}}{{n}_{1}} + \dfrac{{d}_{2}}{{n}_{2}} ⇒ d ' = \dfrac{d}{2} ( \dfrac{1}{{n}_{1}} + \dfrac{1}{{n}_{2}} ) ⇒ d ' = \dfrac{d}{2} ( \dfrac{{n}_{1} + {n}_{2}}{{n}_{1}{n}_{2}} )$