I think the INT function is what you're looking for. It will strip off the
decimal portion of a number, leaving only the Integer portion. See if this
works for you:
=IF(A1>30,INT(A1),0)
This basically states, if A1 (or X) is greater than 30, then return the
integer portion of A1. Since you didn't specifiy what you wanted to happen
if x<30, I just have it returning 0.
Hopefully this will at least point you in the right direction. Post back
with more details if you need more help.
HTH,
Elkar