prep请教思路。

都是PREP模考的题，不明白，请哪位NN给提供一下思路？谢谢。

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

(1) 2 is not a factor of n;  (2) 3 is not a factor of n. (C)

if 2/5 of the students at college of C are business majors, what is the number fo female students at College C? (C)

(1) 2/5 of the male students at colege C are business majors

(2) 200 of the female students at college of C are business majors

If x is an integer, is median of the 5 numbers shown greater than the average of the 5 numbers? (E)

(1) x>6

(2)x is greater than the median of the 5 numbers