A vertical U-tube of uniform inner cross-section contains mercury in both of its arms. A glycerine `("density" 1.3 "gcm"^(-3))` column of length `10 cm `is introduced into one of the arms Oil of density `0.8 "gcm"^(-3)` is poured in the other arm until the upper surface of the oil and glycerine are in the same horizontal level. Find the length of the oil column.
