For a certain set of numbers, if x is in the set, then x - 4 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) -3
c) -6
Choose one.
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 1-4=-3. Similarly, since -3 is in the set, then so must be -3-3=-6. Could 4 and -1 be in the set? Certainly, but we don't know that for sure.   Answer: E.   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>E</result>
For a certain set of numbers, if x is in the set, then x - 4 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) -3
c) -6