What is the difference between the smallest six-digit whole number and the greatest 4-digit number

What's the smallest six-digit whole number possible?
It is 100000

try counting, six digits, right? if you even take 1 off from it, it's no longer six digits, so it is the smallest six-digit whole number possible.

what's the greatest 4-digit number?
It should be 9999
try adding 1 to it, it becomes five-digit number.

So, 9999 is the greatest 4-digit whole number possible.

To get a difference, subtraction.
100000 - 9999 = 90001

Just as a tip, for six digit number, 000000 does not work, as it is same as 0