Why not just do the tests in descending order? Test if they have over 30 days, then if not have they over 20 days, etc.
I think Sorbsbuster is suggesting an elegant alternative here, but to answer you search for an expression:
( days > 20 ) and ( days < 30 )
Will be true only for values from 20 to 30.
Thank you both for responding.
PhilModJunk: I went with your suggestion and it works great.