Calculating the Number of 5-Digit Numbers
There are several rules to consider when calculating the number of 5-digit numbers.

Rule 1: Leading Zero
- A 5-digit number cannot have a leading zero, so the first digit can be any number from 1 to 9.

Rule 2: Remaining Digits
- The remaining four digits can be any number from 0 to 9.

Calculating the Number of Possibilities
To calculate the total number of 5-digit numbers, we need to consider the following:
- There are 9 possibilities for the first digit (1 to 9).
- There are 10 possibilities for each of the remaining four digits (0 to 9).

Calculating the Total
- The total number of 5-digit numbers can be calculated by multiplying the number of possibilities for each digit:
Total = 9 (possibilities for the first digit) * 10^4 (possibilities for the remaining four digits)
Total = 9 * 10,000
Total = 90,000
Therefore, there are a total of 90,000 5-digit numbers in all.