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Find the number of four-letter words that can be formed using the letters of the word PISTON, in which at least one letter is repeated.

Answer
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Hint: In the given question first we have to count the total number of alphabets is the given word PISTON. Then we have to find the number of four-letter words that can be formed with repetition, Number of four-letter words that can be formed without repetition. Then we can calculate the number of four-letter words that can be formed at least with one repetition.

Complete step-by-step solution:
From the given question we can write that:
The number of distinct alphabets in the word PISTON is 6.
Now we have to make several four-letter words
The number of four-letter words that can be formed with repetition will be,
6×6×6×6=1296
Because the total number of letters is six.
Now again according to the question:
The number of four-letter words that can be formed without repetition will be,
6×5×4×3=360
Now, the number of four-letter words that can be formed at least with one repetition will be
(Number of four-letter words that can be formed with repetition) – (Number of four-letter words that can be formed without repetition)
Substitute the value,
1296360=936

Hence, the number of four-letter words that can be formed with the letters in the word PISTON with at least one letter repeated is 936.

Note: In the given question we have to remember the word formula can be formed with one repetition because this formula is used in the given problem i.e., Number of four-letter words that can be formed at least with one repetition = (Number of four-letter words that can be formed with repetition)-(Number of four-letter words that can be formed without repetition).