Fundamental principle of counting states that “if an event can occur in m different ways, following which another event can occur in n different ways, then the total number of occurrence of the events in the given order is m x n.” Let us find the number of 4 letter words, with or without meaning that can be formed out of the letters from the word ROSE, where we could repeat the letters. Since we have to find 4 letter words, draw 4 squares `square square square square` . Keep in mind that repetition is not allowed. The first place can be filled in 4 different ways and the remaining three letters in three different ways, following which the third place can be filled in 2 different ways; following which, the fourth place can be filled in 1 way. Thus, the number of ways in which the 4 places can be filled, by the multiplication principle, is 4 x 3 x 2 x 1 = 24. Hence, the required number of words is 24.Here we can see teh definition of permutation.

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