prep1中有关统计的一道题？？？？

均值和方差以前考的似乎都是一些定义什么的，GMAT数学介绍中好像没有介绍过下面这个问题，我将这些统计知识都还给老师了，麻烦帮忙解决下列问题。

prep1,

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

我是这样想的，假设x为12个中的任一个，那么就有-1.5*0.3<X-8.1<1.5*0.3, 这样可以解出7.65<X<8.55,这时候有11个样本，怎么知道有几个在这个区间里面？