How many 5-digit numbers can be formed using (0-9)?
100 000 combinations
You can have up to 10 combinations for each digit, times the number of... Numbers you want.
So you have
The trick is to realize that a number can not start with a zero!
Now, there are
So, the number of possible five digit numbers is
These are the numbers 10000 to 99999. We could, of course, just have counted them to get