Instead of this system of using fractional question numbers, I would consider giving the questions a simple incrementing integer for identification, and having the order of the questions be stored by other means. A simple system for maintaining order might be a linked list. A table to hold this information could look something like this:
id | next
----+------
1 | 2
2 | 3
3 | 4
4 | NULL
Then, if someone asks you to insert a question between 2 and 3, you would simply have to change it like so:
id | next
----+------
1 | 2
2 | 5
5 | 3
3 | 4
4 | NULL