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Permutations and Combinations question

2020 · Shift 2 · Q32
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Permutations and Combinations question

2020 · Shift 2 · Q32

JEE AdvancedMathematicsPermutations and CombinationsNumerical+4 / −1
In a hotel, four rooms are available. Six persons are to be accommodated in these four rooms in such a way that each of these rooms contains at least one person and at most two persons. Then the number of all possible ways in which this can be done is ..........
Numerical answer
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Correct answer: 1080

The user wants me to solve a combinatorics problem.

Step-by-step Solution:

  1. Analyze the Constraints and Find the Distribution Pattern

    Let the number of persons in the four distinct rooms be n1,n2,n3,n_1, n_2, n_3,n1​,n2​,n3​, and n4n_4n4​. We are given:

    • Total number of persons = 6. So, n1+n2+n3+n4=6n_1 + n_2 + n_3 + n_4 = 6n1​+n2​+n3​+n4​=6.
    • Each room must have at least one person: ni≥1n_i \ge 1ni​≥1 for i=1,2,3,4i=1, 2, 3, 4i=1,2,3,4.
    • Each room can have at most two persons: ni≤2n_i \le 2ni​≤2 for i=1,2,3,4i=1, 2, 3, 4i=1,2,3,4.

    The only possible integer values for nin_ini​ are 1 or 2. Let's find the combination that sums to 6. Let kkk be the number of rooms with 2 persons. Then 4−k4-k4−k rooms will have 1 person. The total number of persons is given by: 2k+1(4−k)=62k + 1(4-k) = 62k+1(4−k)=6 2k+4−k=62k + 4 - k = 62k+4−k=6 k+4=6k + 4 = 6k+4=6 k=2k = 2k=2 This means that exactly two rooms must accommodate two persons each, and the other two rooms must accommodate one person each. The distribution of persons into the four rooms is (2, 2, 1, 1).

  2. Calculate the Number of Ways to Implement this Distribution

    The problem can be solved in two stages:

    • Stage 1: Choose which of the four rooms will be the ones with two persons.
    • Stage 2: Assign the six persons to the four rooms according to this arrangement.
  3. Stage 1: Choosing the Rooms

    We need to select 2 rooms out of the 4 available rooms to accommodate 2 persons each. The remaining 2 rooms will automatically be assigned 1 person each. The number of ways to do this is given by the combination formula: (42)=4!2!(4−2)!=4×32×1=6 ways\binom{4}{2} = \frac{4!}{2!(4-2)!} = \frac{4 \times 3}{2 \times 1} = 6 \text{ ways}(24​)=2!(4−2)!4!​=2×14×3​=6 ways

  4. Stage 2: Assigning the Persons

    Let's consider one specific choice from Stage 1, for example, Room 1 and Room 2 get 2 persons each, and Room 3 and Room 4 get 1 person each. Now we need to assign the 6 distinct persons to these rooms.

    • Number of ways to choose 2 persons for Room 1 out of 6: (62)\binom{6}{2}(26​).
    • Number of ways to choose 2 persons for Room 2 out of the remaining 4: (42)\binom{4}{2}(24​).
    • Number of ways to choose 1 person for Room 3 out of the remaining 2: (21)\binom{2}{1}(12​).
    • Number of ways to choose 1 person for Room 4 out of the remaining 1: (11)\binom{1}{1}(11​).

    The number of ways to assign the persons for this specific room configuration is: (62)×(42)×(21)×(11)=6×52×4×32×2×1=15×6×2×1=180 ways\binom{6}{2} \times \binom{4}{2} \times \binom{2}{1} \times \binom{1}{1} = \frac{6 \times 5}{2} \times \frac{4 \times 3}{2} \times 2 \times 1 = 15 \times 6 \times 2 \times 1 = 180 \text{ ways}(26​)×(24​)×(12​)×(11​)=26×5​×24×3​×2×1=15×6×2×1=180 ways

  5. Calculate the Total Number of Ways

    The total number of ways is the product of the number of ways from Stage 1 and Stage 2. Total ways=(Ways to choose rooms)×(Ways to assign persons)\text{Total ways} = (\text{Ways to choose rooms}) \times (\text{Ways to assign persons})Total ways=(Ways to choose rooms)×(Ways to assign persons) Total ways=6×180=1080\text{Total ways} = 6 \times 180 = 1080Total ways=6×180=1080

    Therefore, there are 1080 possible ways to accommodate the six persons in the four rooms according to the given conditions.

Alternative Method (Grouping and Distributing)

  1. Form groups of people: Divide the 6 distinct people into groups of sizes (2, 2, 1, 1). The number of ways to do this is: 6!2!⋅2!⋅1!⋅1!×12!×12!=7204×12×12=45 ways\frac{6!}{2! \cdot 2! \cdot 1! \cdot 1!} \times \frac{1}{2!} \times \frac{1}{2!} = \frac{720}{4} \times \frac{1}{2} \times \frac{1}{2} = 45 \text{ ways}2!⋅2!⋅1!⋅1!6!​×2!1​×2!1​=4720​×21​×21​=45 ways The division by 2!2!2! twice is because there are two groups of size 2 and two groups of size 1, which are indistinguishable at the grouping stage.

  2. Distribute the groups into rooms: Once the groups are formed (e.g., {P1, P2}, {P3, P4}, {P5}, {P6}), they are distinct sets of people. We need to assign these 4 distinct groups to the 4 distinct rooms. This can be done in 4!4!4! ways. Number of ways to distribute=4!=24\text{Number of ways to distribute} = 4! = 24Number of ways to distribute=4!=24

  3. Total number of ways: Total ways=(Ways to form groups)×(Ways to distribute groups)=45×24=1080\text{Total ways} = (\text{Ways to form groups}) \times (\text{Ways to distribute groups}) = 45 \times 24 = 1080Total ways=(Ways to form groups)×(Ways to distribute groups)=45×24=1080

Both methods yield the same result.

Final Answer: The final answer is 1080.

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