求教3道Prep DS题, DS真的很糟糕，后天就要考了。。。

小lisa

1. For a certain set of n numbers, where n>1, is the average equal to the median?
1) If the n numbers in the set are listed in increasing order, then the difference between any pair of successive numbers in the set is 2
2) The rang of the n numbers in the set is 2(n-1)
答案是A，我选了E（我一上来就把A排除了 (  median的值与n是奇数或偶数没有关系吗？）

2. What is the average height of the n people in a certain group?
1) The average height of the n/3 tallest people in the group is 6 feet 5/2 inches, and the average height of the rest people in the group is 5 feet 10 inches.
2) The sum of the heights of the n people is 178 feet 9 inches.
答案是A，我选了D（知道了sum不可以知道average了吗？）
(这两道都是数理题，哎，就是搞不太明白呀~~)

3. In the fraction x/y, where x and y are positive integers, what is the value of y?
1) The least common denominator of x/y and 1/3 is 6
2) x=1
答案是E，我选了C (不懂，好像觉得如果x=1的话，y不就=6吗？)

ztiamot

以下是引用小lisa在2008-11-3 22:40:00的发言：
1. For a certain set of n numbers, where n>1, is the average equal to the median?
1) If the n numbers in the set are listed in increasing order, then the difference between any pair of successive numbers in the set is 2
2) The rang of the n numbers in the set is 2(n-1)
答案是A，我选了E（我一上来就把A排除了 (  median的值与n是奇数或偶数没有关系吗？）

2. What is the average height of the n people in a certain group?
1) The average height of the n/3 tallest people in the group is 6 feet 5/2 inches, and the average height of the rest people in the group is 5 feet 10 inches.
2) The sum of the heights of the n people is 178 feet 9 inches.
答案是A，我选了D（知道了sum不可以知道average了吗？）
(这两道都是数理题，哎，就是搞不太明白呀~~)

3. In the fraction x/y, where x and y are positive integers, what is the value of y?
1) The least common denominator of x/y and 1/3 is 6
2) x=1
答案是E，我选了C (不懂，好像觉得如果x=1的话，y不就=6吗？)

先说下后两题：

2，1)-->平均身高为：(1/3)*(6 feet 5/2 inches)+(2/3)*(5 feet 10 inches),不需要知道n为多少就可以算出

      2）需要知道n具体是多少，否则算不出

3， 1）不充分。2）不充分。1)+2) 举反例：当x=1时， y可以等于2，也可以等于6. 所以还是不充分。

ztiamot

再说第一题：

等差数列中，平均数等于中位数

刚刚用了一个更麻烦的办法算出的，这个是突然想到的，LZ记这个好了。

[此贴子已经被作者于2008-11-4 0:25:24编辑过]

小lisa

多谢ztiamot大哥! 都明白了！