A U-tube with both ends open to the atmosphere is partially filled with water. Oil, which is immiscible with water, is poured into one side until it stands at a distance of 10 mm above the water level on the other side. Meanwhile, the water rises by 65 mm from its original level (see diagram). The density of the oil is:

From the above diagram,
We get
FE = 10mm
Also, AB = 65 + 65 + 10 = 140 mm = 0.14 m
EC = 65 + 65 = 130 mm = 0.13 m
Pressure at point B
PB = Po + ρoilg(AB)
where Po is the atmospheric pressure.
∴PB = Po + ρoilg(0.14)
Similarly, pressure at point C
PC = Po + ρwaterg(EC)
where ρwater = 1000 kg/m³
PC = Po + ρwaterg(0.13)
Pressure inside the tube from both sides will be the same i.e., PB = PC
∴Po + ρoilg(0.14) = Po + ρwaterg(0.13)
Or ρoilg(0.14) = ρwaterg(0.13)
Density of oil ρoil = 1000 × 0.13 / 0.14 ⟹ ρoil = 928 kgm⁻³