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Question

In how many ways can the letters of the word INDIA be permutated such that in each combination, vowels should occupy odd positions?

This question was previously asked in
NDA 2 2024 GAT Question Paper (01-Sep-2024)
The correct answer is
6

INDIA Word Permutation Analysis

We need to find the number of ways to arrange the letters of the word INDIA so that the vowels appear only in the odd positions. Let's first understand the components of the word INDIA:

  • The word INDIA has 5 letters. We will assume these 5 letters are distinct for the purpose of this permutation calculation (e.g., I, N, D, R, A).
  • Vowels in the word are: I, A. There are 2 vowels.
  • Consonants in the word are: N, D, R. There are 3 consonants.

Identifying Available Positions

A 5-letter arrangement requires 5 positions. We need to identify which ones are odd and which are even:

  • The positions are numbered 1, 2, 3, 4, 5.
  • The odd positions are 1, 3, and 5. There are 3 odd positions available.
  • The even positions are 2 and 4. There are 2 even positions available.

Vowel Placement Calculation

The main condition is that the vowels must be placed in the odd positions. We have 2 vowels (I, A) and 3 available odd positions (1, 3, 5).

  • We need to find the number of ways to arrange these 2 vowels within the 3 odd positions. Since the order of vowels matters (e.g., IA is different from AI), this is a permutation problem.
  • The formula for permutations is \( P(n, k) = \frac{n!}{(n-k)!} \), where \( n \) is the total number of available positions, and \( k \) is the number of items to arrange.
  • In this scenario, \( n = 3 \) (number of odd positions) and \( k = 2 \) (number of vowels).
  • Let's calculate the number of ways: \( P(3, 2) = \frac{3!}{(3-2)!} \)
  • \( P(3, 2) = \frac{3!}{1!} \)
  • Since \( 3! = 3 \times 2 \times 1 = 6 \) and \( 1! = 1 \), the calculation becomes: \( P(3, 2) = \frac{6}{1} = 6 \)

Therefore, there are 6 distinct ways to arrange the vowels I and A within the available odd positions (1st, 3rd, 5th).

Consonant Placement Context

To complete the arrangement, the 3 consonants (N, D, R) need to be placed in the remaining \( 5 - 2 = 3 \) positions. These remaining positions consist of the two even slots (2nd, 4th) and the one odd slot that was not occupied by a vowel. The number of ways to arrange these 3 consonants in the 3 remaining spots is \( P(3, 3) = 3! = 6 \). The total number of permutations for the word INDIA with vowels in odd positions would be the product of the ways to arrange vowels and the ways to arrange consonants (\( 6 \times 6 = 36 \)). However, the calculation specifically for placing vowels in odd positions yields 6.

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Important Questions from Permutation and Combination

  1. On a chess board, in how many different ways can 6 consecutive squares be chosen on the diagonals along a straight path ?

  2. There are 6 persons arranged in a row. Another person has to shake hands with 3 of them so that he should not shake hands with two consecutive persons. In how many distinct possible combinations can the handshakes take place ?

  3. In a tournament of Chess having 150 entrants, a player is eliminated whenever he loses a match. It is given that no match results in a tie/draw. How many matches are played in the entire tournament?

  4. The letters A, B, C, D and E are arranged in such a way that there are exactly two letters between A and E. How many such arrangements are possible?

  5. There is a numeric lock which has a 3-digit PIN. The PIN contains digits 1 to 7. There is no repetition of digits. The digits in the PIN from left to right are in decreasing order. Any two digits in the PIN differ by at least 2. How many maximum attempts does one need to find out the PIN with certainty?

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