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Permutations and Combinations · Question 10

JEE Main 2026 (Online) 28th January Morning ShiftMCQMedium📈 Topic in 23% papers+4−1
1

Let S = \1, 2, 3, 4, 5, 6, 7, 8, 9\. Let x be the number of 9-digit numbers formed using the digits of the set S such that only one digit is repeated and it is repeated exactly twice. Let y be the number of 9-digit numbers formed using the digits of the set S such that only two digits are repeated and each of these is repeated exactly twice. Then,

  1. 56x = 9y
  2. 21x = 4y
  3. 45x = 7y
  4. 29x = 5y

Answer

(2) 21x = 4y

Solution

For x: one digit appears twice and 7 other digits appear once (8 different digits, 9 places).

Choose the repeated digit: 9C1 = 9; choose the other 7 digits from the remaining 8: 8C7 = 8

Arrangements = 9!2!

x = 9 8 9!2 = 36 9!

For y: two digits appear twice each and 5 other digits appear once.

Choose the two repeated digits: 9C2 = 36; the other 5 from the remaining 7: 7C5 = 21

Arrangements = 9!2! 2!

y = 36 21 9!4 = 189 9!

xy = 36189 = 421

21x = 4y

Answer: (2)

JEE Main 2026 (Online) 28th January Morning Shift

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