[求助]求教GWD数学

数据充分题（答案省略）

The ratio of the number of women to the number of men to the number of children in a room is 5:2:7, respectively. What is the total number of people in the room?

(1) The total number of women and children in the room is 12
(2) There are fewer than 4 men in the room

我选的是A，就是第一个条件充分；正确答案是D，就是两个条件各自充分。

我不明白，第二个条件只是给出一个区间，按照这个推算出来的整体人数也是一个区间，而不是一个具体值，不像第一个。那么怎么能够认为（2）也充分呢？

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数据充分题（答案省略）

If y is an integer and y = |x| + x, is y = 0?

(1) x < 0
(2) y < 1

我选的是A，也就是第一个条件自身充分；正确答案是D，也就是两个各自充分。

对于（2），因为题中并没有说 x 是整数，那如果x = 0.1，y = 0.2，虽然满足y < 1但是y 不等于0。

请教！

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这题完全没有思路

If n is an integer greater than 6, which of the following must be divisible by 3?

A. n(n+1)(n-4)
B. n(n+2)(n-1)
C. n(n+3)(n-5)
D. n(n+4)(n-2)
E. n(n+5)(n-6)

正确答案是A，请问为什么？
另外能够被3整除的数，有什么特点吗？判断一个数，能否被n整除，一般的思路应该怎样？
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数据充分题（选项省略）

Was Lisa's annual salary at least twice as much as Julie's annual salary?

(1) Lisa's annual salary was $20,000 more than Julie's annual salary
(2) Lisa's annual salart was less than $40,000

我选的是E，就是两个条件联合也不够充分；正确答案是C，就是两个条件联合就足够充分。

我列的式子：
设Lisa的工资是L；Julie的工资是J
我们求是否 L>=2J

（1）L-J = 20000
（2）L<=40000
联合（1）和（2）
J<=20000
这并不能确定我们要求的L>=2J~

求教！
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数据充分题（选项省略）

What is the median number of employees assigned per project for the projects at Company Z?

(1) 25 percent of the projects at Company Z have 4 or more employees assigned to each project.
(2) 35 percent of the projects at Company Z have 2 or fewer employees assigned to each project.

我选的是E，就是两个联合也不够充分；答案是C，就是两个联合就够充分。

我的思路是，25%的是大于等于4，35%小于等于2，那么40%的就是在2和4之间。通过这个，只能知道大概的分布，但具体的中位数，怎么能通过这个分布来确定呢？

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数据充分题（选项省略）

One kilogram of a certain coffee blend consists of x kilogram of type 1 coffee and y kilogram of type 2 coffee. The cost of the blend is C dollars per kilogram, where C = 6.5x + 8.5y. Is x < 0.8?

(1) y > 0.15
(2) C>=7.30

我选的是C，就是两个联合够充分；答案给的是B，就是只有第二个充分。

求解！