**Problem: **How many 6-letter words can be formed using the letters A,B and C so that each letter appears at least once in the word.

**Solution**:Number of ways 6 letter words can be formed using letters A,B,C so that each letter appears at least once in the word = Number of ways 6 letter word formed using A,B,C – Number of ways 6 letter word formed using any two of A,B,C + Number of 6 letter words formed by using A,B,C. [ using inclusion exclusion principle. ]

= \({3^6} \) – \({\binom{3}{2}}{2^6} \) + \({\binom{3}{1}}{1^6} \)

= 555 [ans]

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