請問數學一題

observer

Sorry I have a data sufficiency question. Anyone please help???

If S is a set of intergers and  5 is in S, is every multiple of 5 in S ?
1. If x is in S, then x+5 is in S.
2. If x is in S, then x - 5 is in S.

the answer is C.
Could anyone explain why? Thank you

himba

如果5在集合S中，则（1）10，15，20，...，　5+5M（M为自然数）在集合S中；（2）0，-5，-10，...，5-5N（N为自然数）在集合S中，由（1）+（2）得5+5i（i为整数）在集合S中，即every multiple of 5 in s　。