求教prep 数学第一套 DS：20,34,187

windywilling

今天揪出这些题不会做，讨论稿汇总上也没有搜到。

贴在这里请NN赐教。题号是按prep破解版标的。

快考试了，哎……在线等NN！

十分感谢！

20.A certain one-day
seminar consisted of a morning session and an afternoon session.  If each of the 128 people attending the
seminar attended at least one of the two sessions, how many of the people
attended the morning session only?

A.3/4 people attended both sessions.

B.7/8 people attended the afternoon session

选B，为啥呢？怎么想都是C……

34.   2927-!-item-!-187;#058&003024

            

During an
experiment, some water was removed from each of 6 water tanks.  If the standard deviation of the volumes of
water in the tanks at the beginning of the experiment was 10 gallons, what was
the standard deviation of the volumes of water in the tanks at the end of the
experiment?

            

 (1) For each tank,
30 percent of the volume of water that was in the tank at the beginning of the
experiment was removed during the experiment.

            

 (2) The average
(arithmetic mean) volume of water in the tanks at the end of the experiment was
63 gallons

这个题汇总上有，但是讲的不甚了了，请NN讲的详细一点吧！

选A。

187.If x, y, and z are integers greater than 1, what is
the value of x + y + z ?

A. XYZ=70

B.X/YZ=7/10

选A。用代入法做出来了，有更好的解法吗？

happYness

第一题 how many of the people attended the morning session only?

看清 ONLY 没有，，画个图，，很EASY的

第二题是 标准差的性质问题，引用下某牛人的回答

D(aX+b) = a^2DX  (DX指X系列值的二阶矩或者说方差)

题说的SD是二阶矩DX的平方根，那么

SD(aX+b) = |a|SD(X)

愁予

1.一共有三种人，只参加上午的，只参加下午的和都参加的，也就是说参加下午的人=只参加下午的+都参加的。单一B就可以了

2.根据标准方差公式可以得出所有都变成原来的0.7倍的话标准方差也是原来的0.7倍。但是只是平均数变化得不出，因为每个平均数还要加各个数在求和。

3.70=2*5*7.可知70有8个因数，1，70，2，5，7，14，35，10.因为三个数都大于一，所以只能是2，5，7.

driftysnail

1.

A.3/4 people attended both sessions.

说明3/4的人参加了下午的会议， 但是上午的会议有多少人参加不确定。

B.7/8 people attended the afternoon session

7/8的人参加了下午的会议，那么1/7的人没有参加下午的会议，但是每个人至少参加了上午或者下午，所以那1/7只参加了上午的会议。注意题目不是问一共有多少人参加了上午的会议，而是问有多少人ZHI参加了上午的会议。

2.偏离的题我还没研究过， 汗，都块考试了， 不过我觉得记得那个公式是不是就可以了？

3.

A. XYZ=70

XYZ=70=2*5*7只有这一种情况

B.X/YZ=7/10

10X=7YZ.如果X=7,则YZ=10满足， 如果X=14, 则YZ=20也满足，就不能确定了。

windywilling

十分感谢以上三位NN，我彻底明白了。

第一题想复杂了@_@……

谢谢~~~

bth

20.
         Certain one-day
            seminar
            consisted of a morning session and an afternoon session. If each 128 ppl attending seminar attended at least one of the 2 sessions, how many of the ppl attended the morning session only?

1. 3/4 ppl attended both sessions
2. 7/8 ppl attended afternoon session

 

Solution:

P(M U A ) = P(A) + P(M) - P(M n A)

We need ( P(M) - P(M n A) )
We know P(M U A) = 128

Condition 1: 3/4 ppl attended both sessions
This is giving us P(M n A). We still don't know P(A)
INSUFFICIENT

Condition 2: 7/8 ppl attended afternoon session
This is exactly what we need
128 = (7/8)(128) + (P(M) - P(M n A))
=> P(M) - P(M n A) = Only morning = 128 - 112 = 16
SUFFICIENT

Answer B

 

34.
        During an experiment, some water was removed from each of 6 water tanks. If the standard deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

 

(1) For each tank, 30 percent of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.

(2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

 

Solution:

Stmt 1:

If an equal amt of water is removed from each tank, the std deviation would also change by same %

=> SD at the end of the expt = 7 (70% of 10)

Sufficient

 

Stmt 2:

Avg at the end of the experiment = 63

No info on amt of water removed..

Insufficient

 

Ans A.

 

187.
        If x, y, and z are integers greater than 1, what is the value of x + y + z ?

A. XYZ=70

B.X/YZ=7/10

 

Solution:

70=2×5×7

X=2, y=5, z=7

So, 2+5+7=14. SUFF.

 

Stem2:

X=7

Yz=10

Y=1, z=10, y=2, z=5, y=-10, z=-1. NOT SUFF.

 

Answer is A.

 

xinxian

Excellent explanation by bth.

谢谢.