请教nn们GWD数学~今天错了几题~~

aruora

数据充分题（选项省略）

From Leland's gross pay of p dollars last week, t percent was deducted for taxes and then s dollars was deducted for savings. What amount of Leland's gross pay last week remained after these two deductions?

(1) p - s = 244
(2) pt = 7,552

我选的是E，也就是两个加起来也不充分；正确答案是C，就是两个加起来就够充分。

不太明白，两个条件合在一起怎么充分了？！

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If the sequence x1, x2, x3, ...., xn,... is such that X1 = 3 and X(n+1) = 2Xn - 1 for n = 1, then X20 - X19 =

A. 2^19
B. 2^20
C. 2^21
D. 2^20 - 1
E. 2^21 - 1

正确答案是A。
这道题我空着没做。。因为不太明白 X(n+1) = 2Xn - 1 for n = 1 是不是说 这个式子只有对n = 1才成立，而其它不成立。
我算的和这个不一样。我算的结果是 2^(n-1)*X1 - n
得到的答案选项里木有。。。

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The average (arithmetic mean) of the 5 positive integers k, m, s, and t is 16, and k < m < r < s < t. If t is 40, what is the greatest possible value of the median of the 5 integers?

A. 16
B. 18
C. 19
D. 20
E. 22

这题半点思路都没。。。。

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In the decimal representation of x, where 0<x<1, is the tenth digit if x nonzero?

(1) 16x is an integer.
(2) 8x is an integer.

正确答案是B，也就是第二个够充分。
我没太读懂问题，什么叫is the tenth digit if x nonzero? (我检查过了，没有打错)

这题怎么考虑啊。。

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What is the remainder when the positive integer x is divided by 8?

(1)When x is divided by 12, the remainder is 5.

(2)When x is divided by 18, the remainder is 11.

正确答案是E，也就是两个联合也不够充分。。。

一遇到这种有余数的题，我就懵。。。恳请nn们赐教~~

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If x, y, and k are positive numbers such that (x / x+y)(10) + (y / x+y)(20) = k and if x < y, which of the following could be the value of k?

A. 10
B. 12
C. 15
D. 18
E. 30

正确答案是D。。
偶哭鸟。。。不会吖。。。。

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                              Favorable                        Unfavorable                    Not sure

Candidate M           40                                    20                                  40

Candidate N            30                                    35                                  35

The table above shows the results of a survey of 100 voters each responded "favorable", or "unfavorable", or "not sure" when asked about their impressions of candidate M and of  candidate N. What was the number of voters who responded "favorable" for both candidates?

(1) The number of voters who did not respond "favorable" for either candidate was 40.

(2) The number of voters who responded "unfavorable" for both candidates was 10.

正确答案是A，也就是条件1就足够充分。

我这道题也没有啥思路。。绕死我了。。

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in2deep

1：题目的意思就是p-pt-s=？

告诉（1）（2）即可。

2：x2-x1=2

x3-x2=2^2

x4-x3=2^3

继续可得x20-x19=2^19

3：5个数总和80。t=40，k < m < r < s，则k=1，m=2，r=16，s=17。

4：tenth digit小数位个位。

（1）告知是1/2，1/4，1/8，1/16。

（2）告知是1/2，1/4，1/8。可知个位非零。

5：29和65两个，足以。穷举法。

6：x=1，y=4，代入法。

7：画韦恩图可得。

aruora

非常感谢！
想再问一下
数列那道题。 Xn+1 = 2Xn - 1 for n = 1，怎样理解？我怎么觉得它的意思是当n =1 的时候那个等式才成立。。如果这样的话，不就不能往下推了么。。

3：5个数总和80。t=40，k < m < r < s，则k=1，m=2，r=16，s=17。
就是想问，我怎样理解才能到这步啊。。