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Question
How many 3-digit numbers can be formed from the digits 1, 2, 3, 4 and 5 assuming that repetition of the digits is allowed?
Solution
There will be as many ways as there are ways of filling 3 vacant 
places in succession with the five given digits.
In succession by the given five digits. In this case, repetition of digits is allowed. Therefore, the unit place can be filled in by any of the given five digits. Similarly, tens and hundreds digits can be filled in by any of the given five digits.
Thus, by the multiplication principle, the number of ways in which three-digit numbers can be formed from the given digits is 5 × 5 × 5 = 125.
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