A U-tube contains two liquids in static equilibrium: Water of density ${\rho }_W=1000 kg/m^3$ is in the right arm, oil of unknown density $\rho$ is the left arm as shown in figure. Measurement give$l = 135 mm$ and $d = 12.5 mm$. The density of oil is :-

The pressure at any level in a liquid is the same in both arms of the U-tube. That is the pressure at the interface sides are the same.

i.e. $P_1=P_2$

or  ${\rho }_{oil}g\left(d+\mathcal{l}\right)={\rho }_{water}g\mathcal{l}$

$$\begin{gathered} {\rho }_{oil}\times 9.8\left(0.0125+0.135\right)=1000\times 9.8\times 0.135 \\ \therefore {\rho }_{oil}=\frac{1000\times 9.8\times 0.135}{9.8\times \left(0.0125+0.135\right)} \\ = \frac{135}{0.1475}= 915 kg/m^3 \end{gathered}$$