\({p_C} = {p_a} + {\rho _{water}}g{h_{water'}}\)

\(where\,{h_{water}} = CE = \left( {65 + 65} \right)mm = 130\,mm\)

\(pressure\,at\,point\,B,{P_B} = {P_u} + {\rho _{oil}}g{h_{oil}}\)

\(where\,{h_{oil}} = AB = \left( {65 + 65 + 10} \right)\,mm = 140\,mm\)

In liquid, Pressure is same at same liquid level,

\({p_B} = {p_c} \Rightarrow {\rho _{oil}}g{h_{oil}} = {\rho _{water}}g\,{h_{water}}\)

\({\rho _{oil}} = \frac{{130 \times {{10}^3}}}{{140}} = \frac{{13}}{{14}} \times {10^3} = 928.57\,kg\,{m^{ - 3}}\)