You start by numbering them 1 through 100.
Then you remove the odd ones, leaving evens 2 through 100.
Then you renumber them 1 through 50.
Then you remove the odd ones, leaving evens 2 through 50.
Then you renumber them 1 through 25.
Then you remove the odd ones, leaving evens 2 through 24.
Then you renumber them 1 through 12.
Then you remove the odd ones, leaving evens 2 through 12.
Then you renumber them 1 through 6.
Then you remove the odd ones, leaving evens 2 through 6.
Then you renumber them 1 through 3.
Then you remove the odd ones, leaving the 2.
Answer: 2
Edwin