The number of 6-letter words, with or without meaning, that can be formed using the letters of the word "MATHS" such that any letter that appears in the word must appear at least twice is:
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When counting words or arrangements with restrictions, break the problem into cases based on the number of times each letter can appear, and then apply the multiplication principle of counting.
Step 1: Identify the letters in the word "MATHS" and count how many times each can be repeated to form a 6-letter word.
Step 2: Use the counting principle to find the total number of valid words where each letter appears at least twice.
Step 3: The total number of such words is calculated as 15.