各位前辈求解一道prep数学题

qigaga

各位前辈 求教一道prep数学题

7.51  8.22  7.86  8.36
8.09  7.83  8.30  8.01
7.73  8.25  7.96  8.53

A vending machine is designed to dispense 8 ounces of coffee into a cup.  After a test that recorded the number of ounces of coffee in each of 1,000 cups dispensed by the vending machine, the 12 listed amounts, in ounces, were selected from the data.  If the 1,000 recorded amounts have a mean of 8.1 ounces and a standard deviation of 0.3 ounce, how many of the 12 listed amounts are within 1.5 standard deviations of the mean?

(A) Four
(B) Six
(C) Nine
(D) Ten
(E) Eleven

一点思路都没得 求助各位大侠

k51

A么？我猜的，根据平均值上下调整一个与1.5有关的数确定范围，我用开方（1000*0.09/12*1.5）这个数调，不知道对不对

qigaga

答案选的是E~

kaji29

within 1.5 sd means 0.3*1.5=0.45
mean is 8.1
within 1.5 is between (8.1-0.45) and (8.1+0.45) OR:
7.65 < x < 8.55
only one doesn't fit, so 12 - 1 = 11
Eleven is the answer.

夜幕如斯

ls正解

qigaga

感谢各位前辈