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Difficulty: 3/102023 NMTC 2023 (QII-62)

Find the number of ways of arranging the letters of the words DANGER, so that no vowel occupies odd place.

Guide / Hint

Hint 1: Identify the 2 vowels (A, E) and 4 consonants (D, N, G, R) in the word DANGER.

Hint 2: Vowels must only be placed in the 3 even positions (2, 4, 6). Choose and arrange them in ways.

Hint 3: Arrange the 4 consonants in the remaining 4 positions in ways, and multiply the results.

Solution

Step 1 (Analyze Letters of the Word): The word DANGER consists of 6 distinct letters: D, A, N, G, E, R.

  • Vowels: A, E (2 vowels)

  • Consonants: D, N, G, R (4 consonants)

  • Position indices: 1, 2, 3, 4, 5, 6

  • Odd places: 1, 3, 5

  • Even places: 2, 4, 6

Step 2 (Apply Constraints): We are given that no vowel occupies an odd place. This means the 2 vowels (A and E) must occupy the even places (2, 4, or 6).

Step 3 (Calculate Seating Arrangements):

  1. Choose and arrange vowels in even places: We choose 2 even places out of 3 and arrange the 2 vowels:

  1. Arrange consonants: The remaining 4 letters (consonants) are arranged in the remaining 4 places:

Step 4 (Calculate Total Product): Using the product rule:

Step 5 (Conclusion): The number of ways of arranging the letters is exactly 144.

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    2023 NMTC 2023 QII-62 - Olympiad Math Olympiad Question | Leminno