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Permutations and Combinations question

2023 · 24 Jan · Shift 2 · Q32
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  5. /2023 · 24 Jan · Shift 2 · Q32

Permutations and Combinations question

2023 · 24 Jan · Shift 2 · Q32

JEE MainMathematicsPermutations and CombinationsMCQ+4 / −1
The number of square matrices of order 5 with entries from the set {0, 1}, such that the sum of all the elements in each row is 1 and the sum of all the elements in each column is also 1, is :
  1. A
    125
  2. B
    150
  3. C
    225
  4. D
    120
View written solutionFree

Correct answer: D

  1. Let the matrix be a 5×55 \times 55×5 matrix with entries from {0,1}\{0,1\}{0,1}.

  2. Given conditions:

    • Sum of elements in each row is 111.
    • Sum of elements in each column is also 111.
  3. Since entries are only 000 or 111, the condition “sum of each row is 1” means:

    • In each row, there is exactly one entry equal to 111 and the remaining 444 entries are 000.
  4. Similarly, “sum of each column is 1” means:

    • In each column, there is exactly one entry equal to 111 and the remaining 444 entries are 000.
  5. Therefore, such a matrix is exactly a permutation matrix of order 555.

    • A permutation matrix is obtained by placing exactly one 111 in each row and each column.
  6. Count such matrices:

    • In the first row, the 111 can be placed in any of 555 columns.
    • In the second row, it must be in one of the remaining 444 columns.
    • In the third row, one of the remaining 333 columns.
    • In the fourth row, one of the remaining 222 columns.
    • In the fifth row, the last remaining column.

    Hence total number is 5⋅4⋅3⋅2⋅1=5!=120.5 \cdot 4 \cdot 3 \cdot 2 \cdot 1 = 5! = 120.5⋅4⋅3⋅2⋅1=5!=120.

  7. Checking options:

    • A: 125125125 ❌
    • B: 150150150 ❌
    • C: 225225225 ❌
    • D: 120120120 ✅

Therefore, the required number of matrices is 120.\boxed{120}.120​.

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