Section B: Statistics [60 marks]

A group of 11 people consists of 6 men and 5 women, 3 of whom are sisters. A committee consisting of six people is to be selected. Find the number of ways the committee can be formed if
  1. (i)

    it consists of exactly two men,

  2. (ii)

    it includes at least one of the sisters.

  3. (iii)

    Given that the chosen committee consists of 2 sisters, Sue and Suzy, together with 3 other men, Muthu, Mark, Michael and 1 other woman, Wina. They are seated at a round table meant for six people. Find the number of possible arrangements if

    one of the men is to be seated between the two sisters,

  4. (iv)

    the two sisters are sitting directly opposite each other.

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Worked solution

5(i)

1. Set up the expression to select exactly 2 men and the remaining 4 members from the women

Since the committee requires exactly 2 men (out of 6), the remaining 6−2=46 - 2 = 4 members must be women (out of 5).

Number of ways=6C2×5C4\text{Number of ways} = {^6C_2} \times {^5C_4}

2. Evaluate the combinations to find the total number of ways

=15×5=75= 15 \times 5 = 75

Answer

7575

5(ii)

1. Set up the expression using the complement method by subtracting the number of ways to form a committee with no sisters from the total number of unrestricted ways.

Number of ways=(116)−(86)\text{Number of ways} = \binom{11}{6} - \binom{8}{6}

2. Evaluate the combinations to calculate the final number of ways.

Number of ways=462−28=434\text{Number of ways} = 462 - 28 = 434

Answer

434

5(iii)

1. Determine the number of ways to form a block consisting of the two sisters and exactly one man between them.

There are 33 men to choose from to sit between the sisters, so there are (31)=3\binom{3}{1} = 3 choices.
For any chosen man, the two sisters can swap places around him in 2!=22! = 2 ways.
Number of ways to form this block = 3×2=63 \times 2 = 6.

2. Treat the block as a single unit and arrange it with the remaining people around the round table.

The block counts as 11 unit. The remaining people (22 men and 11 woman) count as 33 units.
Total units to arrange in a circle = 1+3=41 + 3 = 4.
Number of ways to arrange 44 units in a circle = (4−1)!=3!=6(4 - 1)! = 3! = 6.
Total possible arrangements = 6×6=366 \times 6 = 36.

Answer

3636

5(iv)

1. Seat the two sisters, fixing the circular arrangement

Since it is a circular table, we can fix the position of one sister (e.g., Sue) to remove the rotational symmetry, which gives 11 way.
The other sister (Suzy) must be seated directly opposite her, giving only 11 choice for her seat.

2. Arrange the remaining people

The remaining 44 people (Muthu, Mark, Michael, and Wina) can be arranged in the 44 remaining distinct seats in 4!4! ways.
Total number of possible arrangements =1×1×4!=24= 1 \times 1 \times 4! = 24

Answer

2424

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