A vertical U-tube of uniform cross-section contains mercury in both sides of its arms as shown below. Gylcerine column of 10 cm is introduced into one of its arms. Oil of density 0.8 g/cc is poured into the other arm until the upper surface of the oil and glycerine are at same horizontal level. [ρHg=13.6 g/cc,ρglycerine=1.3 g/cc] The length of oil column is