Height from which the marble falls H=80 cm=0.8 m and
Speed of the marble u=20 cm/s=0.2 m/s
Time taken by the marble to strike the floor T=√2Hg
=√2×0.810=0.4 s
Horizontal distance from the table edge at which the marble will strike the floor
R=u×T=0.2×0.4
=0.08 m=8 cm