1。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) n is not divisible by 2
2)n is not divisible by 3
the answer is C
求思路，谢谢
这道题好像跟GWD还是FF一道题一样的。
（1）n=2k+1
(2)n=3s+1或3s+2
（1）（2)综合,n可能为n=6m+1或n=6m+5,后者也可写成n=6m-1（关于通项如不清楚，请参见此贴：
所以(n-1)(n+1)＝6m(6m+2)=12m(3m+1)，因为m和3m+1必然一奇数一偶数，所以12m(3m+1)可被24整除，故r为0；
或者(n-1)(n+1)＝(6m-2)6m=12m(3m-1)，因为m和3m-1必然一奇数一偶数，所以12m(3m-1)可被24整除，故r为0；
2。if there are more than two numbers in a certain list, is each of the unmbers in the list equal to 0?
1)
the product of any two numbers in the list is equal to 0.
2) the sum of any two numbers in the list is equal to 0
the answer is B, why 1) is not sufficient?
(1)有可能1，0，0也满足条件啊
3. alice's take-home pay last year was the same each month, and she saved the same fraction of her take-home pay each month. the total amunt of money that she had saved at the end of the year was 3 times the amount of that portion of her monthly take-home pay that she did not save. if all the money that she saved lat year was from her take-home pay, what fraction of her take-home pay did she save each month?
1/2; 1/3; 1/4; 1/5; 1/6
the answer is 1/5, why?
设每月saved为X,did not save为Y,则12X=3Y,4X=Y
fraction of her take-home pay did she save each month:X/(X+Y)=X/5X=1/5
4. if d is the standard deviation of x,y,z, what is the standard deviation of x+5,y+5, and z+5
d; 3d; 15d;d+5; d+15
the answer is d. why?
关于标准方差的定理：序列中每一个数都加上一个常数，标准方差保持不变的！！！ | 
