在卡里斯马初级学院(Charisma Junior College)的学生会中,共有 31 名成员,其中包括学生服务(SS)部门的 22 名成员和魅力影响力(CI)部门的 9 名成员。在学院的晨会上,共有 6 名成员值日,两人一组进行。 每天早晨的三项不同值日任务包括:每日启示(Daily Inspirations)、升旗(Flag Raising)以及领唱国歌和领读誓词(Recital of the national anthem and pledge)。除了“每日启示”部分必须由 CI 部门的成员执行外,所有值日任务都可以由任何学生会成员执行。求:

  1. (i)

    所有可能的值日分组总数。

  2. (ii)

    CI 部门成员 Jill 参与值日的值日分组总数。

  3. (iii)

    SS 部门成员 Jack 参与值日的值日分组总数。

  4. (iv)

    由此说明如果第 (i) 部分中的值日分组是随机抽取的,这对学生会成员是否公平,并给出理由。

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完整解法

6(i)

1. Find the number of ways to assign members to the Daily Inspirations duty.

The Daily Inspirations duty requires 2 members, both of whom must be from the CI Wing (which has 9 members).
Number of ways = (92)=36\binom{9}{2} = 36

2. Find the number of ways to assign the remaining members to the other two distinct duties and calculate the total.

The remaining two duties (Flag Raising and Recital) are distinct and require 2 members each. There are no restrictions, so we choose from the remaining 31−2=2931 - 2 = 29 members.
Number of ways for Flag Raising = (292)=406\binom{29}{2} = 406
Number of ways for Recital = (272)=351\binom{27}{2} = 351
Total number of possible duty groupings = 36×406×351=5 130 21636 \times 406 \times 351 = 5\,130\,216

答案

5 130 2165\,130\,216

6(ii)

1. Calculate the total number of possible duty groupings (using the result from part i).

To find the total unrestricted groupings, we choose 2 members from the 9 CI members for Daily Inspirations (DI). Then, from the remaining 29 members (31 total - 2 chosen for DI), we choose 2 for Flag Raising (FR). Finally, from the remaining 27 members, we choose 2 for Recital (R).
Total groupings=(92)×(292)×(272)\text{Total groupings} = \binom{9}{2} \times \binom{29}{2} \times \binom{27}{2}
=36×406×351=5,130,216= 36 \times 406 \times 351 = 5,130,216

2. Calculate the number of duty groupings where Jill is NOT on duty.

If Jill is entirely excluded from the morning duties, we remove her from the available pool. The CI Wing now has 9−1=89 - 1 = 8 members, and the overall available pool (excluding Jill) is 31−1=3031 - 1 = 30 members.

We choose 2 for DI from the remaining 8 CI members:
(82)=28\binom{8}{2} = 28
The remaining available pool is now 30−2=2830 - 2 = 28 members. We choose 2 for FR:
(282)=378\binom{28}{2} = 378
The remaining pool is 28−2=2628 - 2 = 26 members. We choose 2 for Recital:
(262)=325\binom{26}{2} = 325

Groupings without Jill=28×378×325=3,439,800\text{Groupings without Jill} = 28 \times 378 \times 325 = 3,439,800

3. Subtract the groupings without Jill from the total groupings to find the answer.

Using the complement method, the number of groupings where Jill is on duty is the total possible groupings minus the groupings where she is excluded:
Groupings with Jill=5,130,216−3,439,800=1,690,416\text{Groupings with Jill} = 5,130,216 - 3,439,800 = 1,690,416

答案

1,690,416

6(iii)

1. Establish Jack's possible duties and calculate the ways to assign the Daily Inspirations (DI) duty.

Since Jack is in the SS Wing, he cannot be assigned to the Daily Inspirations (DI) duty (which is restricted to CI members). He must therefore be assigned to either the Flag Raising (FR) or the Recital (R) duty.

The DI duty must first be filled by 2 members chosen from the 9 in the CI Wing:
Ways to assign DI=(92)=36\text{Ways to assign DI} = \binom{9}{2} = 36

2. Assign Jack to a duty, choose his partner, and assign the remaining duty to find the total combinations.

Jack can be assigned to either the FR or R duty (22 choices).
Once his duty is chosen, he needs 11 partner from the remaining 2828 members (since 22 are in DI and Jack is 11, leaving 31−2−1=2831 - 2 - 1 = 28): (281)=28\binom{28}{1} = 28.

The remaining duty requires 22 members from the 2727 members left: (272)=351\binom{27}{2} = 351.

Using the multiplication principle:
Total groupings=36×2×28×351=707,616\text{Total groupings} = 36 \times 2 \times 28 \times 351 = 707,616

答案

707,616

6(iv)

1. Establish the values from the earlier parts to compare the number of groupings for a CI member versus an SS member.

Total possible duty groupings (from part i):
N=(92)(292)(272)=36×406×351=5,130,216N = \binom{9}{2}\binom{29}{2}\binom{27}{2} = 36 \times 406 \times 351 = 5,130,216

Groupings where Jill, a CI member, is on duty (from part ii), found by subtracting groupings without her:
N−(82)(282)(262)=5,130,216−(28×378×325)=1,690,416N - \binom{8}{2}\binom{28}{2}\binom{26}{2} = 5,130,216 - (28 \times 378 \times 325) = 1,690,416

Groupings where Jack, an SS member, is on duty (from part iii), found by subtracting groupings without him:
N−(92)(282)(262)=5,130,216−(36×378×325)=707,616N - \binom{9}{2}\binom{28}{2}\binom{26}{2} = 5,130,216 - (36 \times 378 \times 325) = 707,616

2. Compare the number of groupings to justify the comment on fairness.

Since 1,690,416>707,6161,690,416 > 707,616, a CI member is involved in more than double the possible duty groupings compared to an SS member. If a grouping is picked uniformly at random, a CI member inherently has a higher probability of being chosen for duty. Thus, the system is not fair.

答案

It is not fair. A member of the CI Wing (like Jill) appears in 1,690,4161,690,416 possible groupings, whereas a member of the SS Wing (like Jack) appears in only 707,616707,616 possible groupings. Since a CI member has a significantly higher chance of being picked than an SS member, picking groupings at random is unfair.

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