[求助]请帮忙四道数学题！

 1. If a and b are integers, is b even?

(1)
                  3a + 4b is even.

(2)
                  3a + 5b is even.

我选E，可答案是C，不懂是为什么？

2.

When tossed, a certain coin has equal probability of landing on either side.  If the coin is tossed 3 times, what is the probability that it will land on the same side each time? A.1/8   B. 1/4     C. 1/3   D.3/8    E.1/2

3.

If xyz
                            ≠ 0, is x (y + z) ≥ 0?

(1)   │y + z│ = │y│ + │z│

(2)
                                  │x + y│ = │x│ + │y│

4.

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.

这题我选E，可答案说是C，为什么？

If xyz
                    ≠ 0, is x (y + z) ≥ 0?

(1)   │y + z│ = │y│ + │z│

(2)
                          │x + y│ = │x│ + │y│

4.

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.

这题我选E，可答案说是C，为什么？

When tossed, a certain coin has equal probability of landing on either side.  If the coin is tossed 3 times, what is the probability that it will land on the same side each time? A.1/8   B. 1/4     C. 1/3   D.3/8    E.1/2

3.

If xyz
                ≠ 0, is x (y + z) ≥ 0?

(1)   │y + z│ = │y│ + │z│

(2)
                      │x + y│ = │x│ + │y│

4.

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.

这题我选E，可答案说是C，为什么？

If xyz
        ≠ 0, is x (y + z) ≥ 0?

(1)   │y + z│ = │y│ + │z│

(2)
              │x + y│ = │x│ + │y│

4.

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.

这题我选E，可答案说是C，为什么？

一起来讨论吧。我的思路如下：

1。选C 因为（1）只能推出a是偶数，(2)a和b都不能确定

  （1）+（2）连个条件和起来，可推出（3a+5b）-（3a+4b）=b 偶数-偶数，当然还是偶数

2。连续三次都是一个面，有两种可能，正或反，每种概率都是1/8，所以答案是1/4，选B

3。条件（1）可推出y和z同号，（2）可推出x和y同号，（1）+（2）可推出x,y,z同号，x和y+z同号，积必然大于0

4。（1）说25%的工程要4个或4个人以上去做 （2）说35%的工程要2个或少于2个人去做

    （1）+（2）可推出，40%的工程需要3个人去做。一个工程按100%算的话，median number 在那个50%的位置。所以，选C

有不对的地方，欢迎指出来，谢谢：）