A U-tube having a horizontal arm of length $20 \mathrm{cm}$, has a uniform cross-sectional area $1 {\mathrm{cm}}^2$. It is filled with water of volume $60 \mathrm{cc}$. The volume of a liquid of density $4 \mathrm{g} {\mathrm{cc}}^{-1}$ required to be poured in one arm of the U-tube so that no water is left in the horizontal arm of the tube is (take $\mathrm{g}=9.8 \mathrm{m} {\mathrm{s}}^{-2}$)