急请教几道题，感谢回答

lianghao

1. A certain characteristic in a large population has a distribution that is symmetric about the mean m.  If 68 percent of the distribution lies within one standard deviation d of the mean, what percent of the distribution is less than m +
    d ?

A.    16%

B.     32%

C.     48%

D.    84%

E.     92%

2. A school administrator will assign each student in a group of n students to one of m classrooms.  If 3 < m < 13 < n, is it possible to assign each of the n students to one of the m classrooms so that each classroom has the same number of students assigned to it?

(1)   It is possible to assign each of 3n students to one of m classrooms so that each classroom has the same number of students assigned to it.

(2)   It is possible to assign each of 13n students to one of m classrooms so that each classroom has the same number of students assigned to it.

3. Is the hundredth digit of the decimal d greater than 5?

(1) The tenths digit of 10d is 7

(2) The thousandth digit of d/10 is 7.

我没太懂这个decimal是啥意思。选D。

4. Tanya prepared 4 different letters to be sent to 4 different addresses. For each letter, she prepared an envelope with its correct address. If the 4 letters are to be put into the 4 envolopes at random, what is the probability that only 1 letter will be put into the envelope with its correct address?

A. 1/24

B. 1/8

C.1/4

D/1/3

E/3/8

(1)   It is possible to assign each of 3n students to one of m classrooms so that each classroom has the same number of students assigned to it.

(2)   It is possible to assign each of 13n students to one of m classrooms so that each classroom has the same number of students assigned to it.

3. Is the hundredth digit of the decimal d greater than 5?

(1) The tenths digit of 10d is 7

(2) The thousandth digit of d/10 is 7.

我没太懂这个decimal是啥意思。选D。

4. Tanya prepared 4 different letters to be sent to 4 different addresses. For each letter, she prepared an envelope with its correct address. If the 4 letters are to be put into the 4 envolopes at random, what is the probability that only 1 letter will be put into the envelope with its correct address?

A. 1/24

B. 1/8

C.1/4

D/1/3

E/3/8

(1)   It is possible to assign each of 3n students to one of m classrooms so that each classroom has the same number of students assigned to it.

(2)   It is possible to assign each of 13n students to one of m classrooms so that each classroom has the same number of students assigned to it.

3. Is the hundredth digit of the decimal d greater than s?

(1) The tenths digit of 10d is 7

(2) The thousandth digit of d/10 is 7.

我没太懂这个decimal是啥意思。选D。

4. Tanya prepared 4 different letters to be sent to 4 different addresses. For each letter, she prepared an envelope with its correct address. If the 4 letters are to be put into the 4 envolopes at random, what is the probability that only 1 letter will be put into the envelope with its correct address?

A. 1/24

B. 1/8

C.1/4

D/1/3

E/3/8

[此贴子已经被作者于2009/9/6 22:43:10编辑过]

ARCHER420

4. 1/4*1/2*1=1/8

其他太难了 1，蒙B 32% 2.E蒙的 3条件不全吧，那个s应该是个数字吧 要不底下怎么没出现s呢？

lianghao

以下是引用ARCHER420在2009/9/6 20:24:00的发言：

4. 1/4*1/2*1=1/8

其他太难了 1，蒙B 32% 2.E蒙的 3条件不全吧，那个s应该是个数字吧 要不底下怎么没出现s呢？

第四题答案给的是1/3。

大家帮忙看看难题啊。万分感谢！

lianghao

以下是引用ARCHER420在2009/9/6 20:24:00的发言：

4. 1/4*1/2*1=1/8

其他太难了 1，蒙B 32% 2.E蒙的 3条件不全吧，那个s应该是个数字吧 要不底下怎么没出现s呢？

哦第三题是我抄题的时候抄错了，不是s，是5.。。。。。。改过来了

lenchohu

1，是68%＋16%＝84％  d吧
2.实际就是想问m是不是n的一个factor, m是3n的factor推不出m是n的factor, 比如，m=6,n=14, 根据2），m是13n的factor,因为m小于13，大于3, 在这个区间之间没有数是13的factor或是13倍数，推出，m定是n的一个factor,选b吧
3就是问你一个小数的百位是不是7；1，2都成立，选d
4, 这题好恶心，我一个一个画的，应该是1/3，我也没啥好办法，帮你顶一下。

dshimuro

第4題是 1/3.

信封A,B,C,D;信件a,b,c,d.考虑只有d放对了的情况,一共两种:A-b,B-c,C-a; A-c,B-a,C-b.

   只有一封信放对的情况就有4*2=8种.

   四封信放四个信封里,一共有4!=24种放法.

   所求概率=8/24=1/3.选D.

LLsheep

题1的理解重点在symmetric和standard deviation，对称分布下，平均数是m，标准差是d

我把m假设为坐标轴上一个左右对称的抛物线的顶点，比如点（m，0）

68%分布在一个标准差内，即68%的分布在m-d<x<m+d的区域，现在问x<m+d的有多少

因为对称，m左右各50%，68%+（1-68%）/2=84%

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题2选B，题目可理解为问n/m是否整数，这样才能把全部学生平分到每个班级

（1）表示3n/m是整数，m为3的倍数（6/9/12)均可，可以自己举例，n/m不一定为整数，不充分。

（2）13n/m是整数，因为m<13，故n/m必为整数，充分。

——————————————————————————

题3是没理解意思吧，一个小数的百分位，即小数点右起第二位，是否大于5，例如小数0.XYZ中，Y>5?

（1）（2）分别给出的等式可以得出Y-7，各自充分

————————————————————————————

假想4个信封为4个固定的座位，4封信分别坐进4个座位，则有4!=24种组合形式

只有一封信和一个信封配对成功，则剩下3个信封错误排列的组合为2种

4*2/24=1/3

lianghao

谢谢好心人的解答！大家这么热心！！第一题D，第二题B，第三题D,第四题D。祝各位考试都取得好成绩！