A vertical U-tube of uniform inner cross – section contains mercury in both sides of its arms . A glycerine (density =1.3g/cm3 ) column of length 10 cm is introduced in to one of its arms . Oil of  density 0.8g/cm3  is poured into the other arm until the upper surfaces of the oil and glycerine are in the same horizontal level . Find the length of the oil column . Density of mercury = 13.6g/cm3