Telephone numbers

How many 7-digit telephone numbers can we put together so that each number consists of different digits?

Correct result:

n =  544320

Solution:

n=9 9 8 7 6 5 4=544320n=9 \cdot \ 9 \cdot \ 8 \cdot \ 7 \cdot \ 6 \cdot \ 5 \cdot \ 4=544320



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