GMAT Prep : Numbers

[a.r.deepak]

For a certain set of numbers ,if X is in the set X -3 is also in the set.
If the number 1 is in the set , which of the following must also be in the set ?

1. 4
2. -1
3. -5



[spoiler]OA: -5[/spoiler]

[a.r.deepak]

Doubt :

Why cannot 4 be in the same set ?
1 = x -3 ; X = 4