[Man50D]

I thought one of the provisions of H.R. 25 is that if it passes, it nullifies the 16th Amendment, and abolishes the IRS.

You're partially correct. The Fair Tax Act will abolish the IRS but it can't repeal the 16th Amendment as that requires a different process. There is a separate piece of legislation(House Joint Resolution 16) for repeal. Passage of The Fair Tax requires only a majority vote in Congress. Repealing an amendment requires ratification of three fourths by each state legislature.

[wastedyears]

So I would imagine that the 16th should be taken care of before the Fair Tax would be enacted.

[phil_will1]

“Repealing an amendment requires ratification of three fourths by each state legislature.”

Almost. Enacting or repealing a Constitutional amendment requires ratification by 3/4 of the states (38 right now) by a simple majority in both houses. It also requires ratification by both houses of the US Congress, and I think that is by 2/3. Interestingly enough, the President does not sign.