# What is the formula to get 4 digit combinations from numbers 0-9?

Oct 17, 2016

Please read the explanation, because the answer is either $10 , 000$ or $5 , 040$.
Since there are $10$ choices for each digit, the number of $4$- digit combinations is given by
$10 \cdot 10 \cdot 10 \cdot 10 = 10 , 000$,
UNLESS using a digit means it cannot be used again. In that case, the number of $4$-digit combinations is given by
$10 \cdot 9 \cdot 8 \cdot 7 = 5 , 040$.