Two married couples, two single adults and two children formed a team of 8 to take part in a series of games.

  1. (a)

    In the first game, the team sits in a circle. Find the number of arrangements that can be formed if each married couple must be seated together.

  2. (b)

    A group of three people are to be selected from the team for the second game. Find the number of different groups that can be formed if there must not be a married couple in the group.

  3. (c)

    In the third game, each team member selects a unique number from the set {1, 2, ..., 8}. Find in number of different ways this can be done if the numbers selected by the children are both greater than the numbers selected by the two single adults.

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Worked solution

5(a)

1. Determine the total number of entities to arrange by grouping the couples.

Since each married couple must be seated together, we treat each couple as a single unit.
This gives us:

  • 2 couple units
  • 2 single adults
  • 2 children
    Total entities =2+2+2=6= 2 + 2 + 2 = 6 units.

2. Find the number of ways to arrange the units in a circle.

The number of ways to arrange nn units in a circle is (n−1)!(n - 1)!.
For 6 units: (6−1)!=5!=120(6 - 1)! = 5! = 120 ways.

3. Multiply by the internal arrangements of the couples to find the total number of ways.

Within each couple unit, the two people can switch places in 2!2! ways. Since there are 22 couples, we multiply by 2!2! twice.
Total number of arrangements =5!×2!×2!=120×2×2=480= 5! \times 2! \times 2! = 120 \times 2 \times 2 = 480

Answer

480480

5(b)

1. Find the total number of ways to choose a group of 3 from 8 people without restrictions.

Total unrestricted groups = (83)\binom{8}{3}
(83)=8×7×63×2×1=56\binom{8}{3} = \frac{8 \times 7 \times 6}{3 \times 2 \times 1} = 56

2. Find the number of ways to choose a group that contains a married couple.

To form a group of 3 with a married couple, we must choose 1 of the 2 couples (which takes up 2 spots), and then choose 1 person from the remaining 6 people.
Number of such groups = (21)×(61)\binom{2}{1} \times \binom{6}{1}
2×6=122 \times 6 = 12

3. Subtract the number of restricted groups from the total to find the number of valid groups.

Number of groups without a married couple = 56−12=4456 - 12 = 44

Answer

44

5(c)

1. Select 4 numbers for the children and single adults, and determine their valid assignments.

We first choose 44 numbers out of the 88 available for the 22 children and 22 single adults: (84)\binom{8}{4} ways.

For any set of 44 chosen numbers, the 22 largest must go to the children and the 22 smallest must go to the single adults.
The children can be assigned the 22 largest numbers in 2!2! ways.
The single adults can be assigned the 22 smallest numbers in 2!2! ways.

Ways to assign these 44 people =(84)×2!×2!=70×2×2=280= \binom{8}{4} \times 2! \times 2! = 70 \times 2 \times 2 = 280

2. Assign the remaining numbers to the married adults and calculate the total.

The remaining 44 numbers can be freely assigned to the 44 married adults in 4!4! ways.

Total number of ways =280×4!=280×24=6720= 280 \times 4! = 280 \times 24 = 6720

Answer

67206720

AI analysis — may contain mistakes.

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