[求助]GWD MATH题两道

Q9:

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

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Q10:

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

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Q9:

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

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Q10:

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

我觉得

1、B

首先因为一次拿一个，排成排，所以我觉得应该考虑顺序（如果不考虑顺序，应该也可以，那么后面也不能考虑顺序，要一致）

然后总的有=4×3×2×1 种可能

两红在中间 就是   白红红白

有C(2 1)*C(2 1)*1*1=4   就是先从两白拿1白，再从两红拿1红，然后剩下的红，再剩下的白

所以4/24=1/6

不知道答案对不对，请指正

2、-1在其中所以，-1+2=1 在其中 ， 1+2+2=5在其中

但是不能推-3，因为原文说T在其中，T+2在其中，没说t+2在其中能推出t在其中，就是说原命题成立，逆命题不一定成立，所以选D

Right Answer 

Herzlichen Glueckwunsch!

B,D依次是正确答案。

对于第一题我困惑的是：在分母为24的总排列种数里，我怎么认为白红红白的排列有P(2 2)*P(3 3)种呢?(红的在中间两种，然后把两红的看作一个整体再与两白排列P(3 3) )