Prep math求助大牛

weiqite

1. All of the stocks on the counter market are designated by either 4 or 5 letter code that is created by using the 26 alphabet. which of the following gives the maximun number of different stocks that can be designated with the codes?

Answer: 27(26)^4

2. The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching TV. The distribution of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of that Pat, a resident of Town X, watched TV last week was between 1 and 2 standard deviation below the mean. Which of the following could be the number of hours that Pat watched TV last week?

Answer: 12

请教这2道题目怎么考虑得到这个答案的？谢谢！

sys1010

不是NN。

1.by either 4 or 5 letter code ：如果是4 letter, 就是26^4，如果是5 letter， 就是26^5，总共有26^4+26^5，就是27*26^4.

2.均值是21，方差是6，因为Pat看电视时间 between 1 and 2 standard deviation below the mean，也就是在9到15的范围内。

jockey829

LS:

为什么在9-15间呢...

sys1010

低于均值一到两个方差，就用21分别减去6和6*2。

jockey829

哎呀...谢谢...我理解成...没法说了..完全拧巴着...

weiqite

恩，谢谢，原来也没有看懂题意。

谢谢~~