How many 5-digit numbers can be formed using

Explanation:
You can have up to 10 combinations for each digit, times the number of... Numbers you want.
So you have ${\displaystyle n={10}^5=100000}$.

Step 1
The trick is to realize that a number can not start with a zero!
Now, there are ${\displaystyle {10}^5}$ ways in which the digits 0-9 can be chosen for the five places of a five digit number. Out of these, ${\displaystyle {10}^4}$ start with zero (once we start with 0, there are only 4 slots to fill, where we have 10 choices each).
Step 2
So, the number of possible five digit numbers is
${\displaystyle {10}^5-{10}^4=9\times {10}^4=90000}$
These are the numbers 10000 to 99999. We could, of course, just have counted them to get ${\displaystyle 99999-10000+1=90000}$.