for 2 folks, chans 2 hav same birthdae=1/365 . . . or chans dont hav=364/365 . . . 3 folks...chans folk3 DONT hav same as eether av first 2=363/365 . . . over all chans=363*364/(365^2)=99.18% . . . 4 folks...chans=362*363*364 / (365^3) ... =47,831,784/48,627,125=0.983644087 . . . or 98.4%