Since the numbers must begin with 231, we can disregard the first 3 numbers and only consider the last 4. First, let's find how many combinations there are without any restrictions.
There are 10 digits to choose from, and there are 4 numbers, we have:
[tex]10^4=10000[/tex] combinations
Next, let's subtract any combinations that do not contain any 9's. If the number does not have have 9's, that means there are only 9 digits to choose from, so we remove:
[tex]9^4 = 6561[/tex] combinations.
10000 - 6561 = 3439
The answer is 3439.
