请教几道数学难题

实在做不出来，请指教解题思路：

3-Q16: 不会

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

(1)     2 is not a factor of n.

(2)     3 is not a factor of n.

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3-Q36: 不会

If M is the least common multiple of 90, 196, and 300, which of the following is NOT a factor of M?

A.      600

B.       700

C.      900

D.      2,100

E.       4,900

原来已经有人问过了，不问了