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  4. How many four digit PIN numbers can be made using only the digits 2, 3 and 4 and using each of these digits at least once in each PIN number?

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Q. How many four digit PIN numbers can be made using only the digits 2, 3 and 4 and using each of these digits at least once in each PIN number?

Permutations and Combinations

Solution:

2234 or 2334 or $2344 \Rightarrow 3 \cdot \frac{4 !}{2 !}=36$