How many different seven-digit phone numbers begin with 231and contain at least one 9?

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:

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:

combinations.

10000 - 6561 = 3439

The answer is 3439.