In the Student Council of Charisma Junior College, there are a total of 31 members, comprising 22 members in the Student Services (SS) Wing and 9 members in the Charismatic Influence (CI) Wing. At the college's morning assembly, a total of 6 members are on duty, carried out in pairs. The three different duties every morning consist of the Daily Inspirations, the Flag Raising and the Recital of the national anthem and pledge. All duties can be carried out by any student councillor except the Daily Inspirations component, which must be carried out by members of the CI Wing. Find

  1. (i)

    the total number of possible duty groupings.

  2. (ii)

    the total number of duty groupings where Jill, a member of the CI Wing, is on duty.

  3. (iii)

    the total number of duty groupings where Jack, a member of the SS Wing, is on duty.

  4. (iv)

    Hence comment on whether it is fair to the Student Council members if the duty groupings in part (i) were picked at random, justifying your answer.

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Worked solution

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

Answer

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

Answer

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

Answer

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.

Answer

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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