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crazykrans
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If n is a multiple of 5, and n=ppq, where p and q are primes, then either p or q must be 5.
Q: which of the following MUST be a multiple of 25?
Well, we know that either p or q is a multiple of 5, but:
1) we don't know which one it is; and
2) neither one is itself a multiple of 25, since then it couldn't be prime.
Therefore, the only answer that MUST be a multple of 25 is:
d) ppqq
since either pp or qq is definitely a multiple of 25.
All of the other choices COULD be multiples of 25, but that's not what the question is asking.
