For a certain set of numbers, if x is in the set, then x - 2 is also in the set. If the number 1 is in the set , which of the following must also be in the set ?
a) -4
b) -1
c) -5 Choose the correct choice from the following choices
A) a only, B) b only, C) c only, D) a and b only E) b and c only
Notice that the question asks which of the following MUST be in the set, not COULD be in the set.   Since 1 is in the set, then so must be 1-2=-1. Similarly, since -1 is in the set, then so must be -1-3=-4. Could 4 and -1 be in the set? Certainly, but we don't know that for sure.   Answer: D.   Regarding your question: we don't know which is the source integer in the set, if it's 1, then 4 won't be in the set but if the source integer is say 7, then 4 will be in the set. So, 4 may or may not be in the set.
<result>D</result>
For a certain set of numbers, if x is in the set, then x - 2 is also in the set. If the number 1 is in the set , which of the following must also be in the set ?
a) -4
b) -1
c) -5