急问几道数学题

alljobest

A gardener is going to plant 2 red rosebushes and 2 white rosebushes.  If the gardener is to select each of the bushes at random, one at a time, and plant them in a row, what is the probability that the 2 rosebushes in the middle of the row will be the red rosebushes?

 

A.      1/12

B.      1/6

C.      1/5

D.     1/3

E.      1/2

Answer: B

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A set of numbers has the property that for any number t in the set, t + 2 is in the set.  If –1 is in the set, which of the following must also be in the set?

 

I.                     -3

II.                   1

III.                  5

 

 

A.          I only

B.          II only

C.          I and II only

D.          II and III only

E.          I, II, and III

Answer: D 

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In a corporation, 50 percent of the male employees and 40 percent of the female employees are at least 35 years old.  If 42 percent of all the employees are at least 35 years old, what fraction of the employees in the corporation are females?

 

A.      3/5

B.      2/3

C.      3/4

D.     4/5

E.      5/6

Answer：D

请教各位大牛如何解这3道题！谢谢！

wzy4223

第一题：（P22*P22）/P44=(2*2)/4!=1/6

第二题：-1在集合中，则-1+2=1在集合中，1+2=3在集合中，3+2=5在集合中，总之，-1以后的奇数都在集合中。而不能保证小于-1的奇数也在这个集合中。

第三题：设Female的比例是x，则Male的比例是（1-x）,由题可知

50%（1-x）+40%x=42%, 解出x=4/5

alljobest

非常感谢! 全明白了!