Permutations and Combinations 3 Question 9

9. Words of length 10 are formed using the letters A, B, C, D, E, F, G, H, I, J. Let x be the number of such words where no letter is repeated; and let y be the number of such words where exactly one letter is repeated twice and no other letter is repeated. Then, y9x=

(2017 Adv.)

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Answer:

Correct Answer: 9. (5)

Solution:

  1. x=10 !

y=10C1×9C8×10!2!=10×9×10!2y9x=102=5



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