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1. Ten different letters of alphabet are given, words with 5 letters are formed from these given letters. Then, the number of words which have at least one letter repeated is:

Solution:
Number of words which have at least one letter replaced,
= Total number of words - total number of words in which no letter is repeated.
= 105 – 16P5
= 100000 − 30240
= 69760

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