am tearing my hair out over a question to do with acceleration due to gravity....

a stone is dropped from rest, from a height of 20m; 0.6 seconds later a marble is thrown with a velocity of 8m/s from the same height. when does the marble overtake the stone..?(assuming gravity to be 10m/s~2.)

ive spent so much time on this but am really confused about the time factor..

i know that it overtakes when the displacement is the same so when

ut+1/2at^2=ut+1/2at^2 but im confused that one has a head start of .6 sec....

ive tryed working out that the stone has a 1.8m displacement after .6 seconds and added that to the formula before making both sides equal but what i end up with is

(stone)1.8m + 5t^2 =(marble) 8t + 5t^2

dont know what im doing wrong.... anyone that can help out id be greatly appriciative...