Answer:
90,000
Explanation:
The first 5 digit number is 10,000, and the last 5 digit number is 99,999, so everything in between these numbers must be a 5 digit number.
You can find the number of numbers in between by subtracting them.
99,999 - 10,000 = 89,999
However, when subtracting, you leave out one of the numbers, so you add 1, which gives 90,000.
This can be shown with a smaller problem, such as how many 1 digit numbers are there:
1, 2, 3, 4, 5, 6, 7, 8, 9 - 9 numbers
However, when you subtract the biggest and smallest numbers:
9-1 = 8
As you see, you are missing on of the numbers because you didn't add one of the numbers in the problem.