That's called Arabic numerals so I think the answer is Arabians....
There are no 3 digit numbers between 0 and 9 because 0 and 9 are 1 digit numbers.
9
9
9
9+0=9?
There are 10000 such codes. Each of the numbers 0-9 can be in the first position. With each such first digit, each of the numbers 0-9 can be in the second position. With each such pair of the first two digits, each of the numbers 0-9 can be in the third position. etc.
9
-9
The sum of these numbers is 38. There are 8 numbers. The mean is thus:38/8 = 4.75
0 1 8
9
There are infinitely many numbers between 0 and 9. One such is 1.7.