Please help! 两道Prep数学题

1. A certain list of 100 data has an average of 6 and a standard deviation of d, where d is positive, which of the following pairs of data, when added to the list, must result in a list of 102 data with the standard deviation less than d?

A -6 and 0

B 0 and 0

C 0 and 6

D 0 and 12

E 6 and 6

Answer: E    please help!, i always cannot figure out the questions related to SD...........

2. If 500 is the multiple of 100 that is the closest to X and 400 is the multiple of 100 that is closest to Y, which multiple of 100 is closest to X + Y?

1. X < 500

2. Y< 400

answer: E    totally dun understand what this question means..............

第一题我能解答...

ABC不能选因为这两个数值影响了平均值，从而在整体上影响了d，无法判断结果。

D虽然没有影响平均值，但是因为不知道另外100个数的范围，而d是未知的。举例而言，如果这100个数有50个是5，50个是7，d应该等于1，那么加入0和12，d一定>1。

选E将不影响平均值。而在计算标准差时，新增的两个数据偏离均为0，则会削弱数据的偏离即标准差。

第二题同看不懂。再研究一下。

好像看懂了...

若500是最接近X的100的倍数，400是最接近Y的100的倍数，哪个数是最接近X+Y的100的倍数？

取X=451,Y=351，X+Y=802,X=499,Y=399,X+Y=898，则有800和900两个100的倍数，无法唯一确定。选E