11th Class Mental Ability Permutations and Combinations Question Bank Question - Permutation and Combination

  • question_answer
    Find the number of ways of selecting 4 letters from the word EXAMINATION.

    A) 136

    B) 126

    C) 252

    D) 525

    E) None of these

    Correct Answer: A

    Solution :

    Explanation Option (a) is correct. There are 11 letters in the given word of which 2 are A,s, 2 are I?s 2 are N?s and the remaining 5 letters are different. Thus we have 11 letters of 8 different kinds viz, (A, A), (I, I), (N, N) E, X, M, T, O. A group of 4 letters can be classified as follows: (i)  Two alike of one kind and two alike of another kind. (ii) Two alike and the other two different. (iii) All four different. In case I, the number of ways = \[^{3}{{C}_{2}}=3\] Incase II, the number of ways = \[^{3}{{C}_{1}}{{\times }^{7}}{{C}_{2}}=63\] In case III, the number of ways = \[^{8}{{C}_{4}}=70\] Hence, the required number of ways \[=3+63+70=136\].


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