How many different seven-digit phone numbers begin with 231and contain at least one 9?
1 answer:
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.
There are 10 digits to choose from, and there are 4 numbers, we have:
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:
10000 - 6561 = 3439
The answer is 3439.
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