这道prep数学该怎么做？

cooperqi

If a certain sample of data has a mean of 20.0 and a standard deviation of 3.0, which of the following values is more than

2.5 standard deviations from the mean?

 

(A) 12.0

(B) 13.5

(C) 17.0

(D) 23.5

(E) 26.5

答案是A

这道题该怎么做呀？请指点。。。急等

lixinli

20+ or -2.5*3=27 or 12.5 所以选a

seaskyangs

mean=20 standard deviation 3,
first you can find the range(more than 2.5 standard deviations)
(20-2.5*3,20+2.5*3) =(12.5,27.5)
(A) 12 <12.5, that is why we choose A
good luck

cooperqi

非常感谢

cooperqi

我想想还是没想通，为什么要用3×2.5呢？

yujingmm

mean=20, std deviation=3
2.5 std deviation from the mean: 意思是某值距离平均值(20)有2.5个std deviation那么远，则该某值可以在平均数的左边或右边并且距离有2.5个std deviation. --> 左边：20-2.5*3=12.5 or 右边：20+2.5*3=27.5
ask "more than 2.5 std deviation from the mean" --> 即比这个距离还要大，则要比12.5小，或比27.5大。答案只有12符合。
希望可以帮到lz。