请教两道PREP数学题！

PART1的159：

If n is a positive integer and r is the remainder when (n-1)(n+1)is divided by 24, what id the value of r?

(1) n is not divisible by 2

(2) n is not divisible by 3

答案选C

170：

If n and t are positive integers, what is the greatest prime factor of the product nt?

(1) the greatest common factor of n and t is 5

(2) the least common multiple of n and t is 105

答案：B

这两道题我一点思路都没有，不知道如何下手，请高手指教啊！

快考试了，发现有的PREP题还是有点难度，容易把人绞糊涂，挺担心的，难道正式考试的题都是这样？