I computed the answer to be $10 \cdot 9 \cdot 8 \cdot 7$, but apparently we had to do $9 \cdot 9 \cdot 8 \cdot 7$.
But there are $10$ digits between $0$ and $9$, right? Can someone explain to me how this is possible?
[Math] How many integers between $1000$ and $9999$ have distinct digits
combinatorics
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Best Answer
Hint: The first of your four digits can't be $0$ if the number is between $1000$ and $9999$.