Water and oil are poured into the two limbs of a U-tube containig mercury. The interface

Water and oil are poured into the two limbs of a U-tube containig mercury. The interface of the mercury and the liquids are at the same height in both limbs. Determine the height of the water column`h_(1)` if that of the oil `h_(2)=20cm`. The density of the oil is `0.9`.