Phone numbers are 10 digits long. How many possible phone numbers are there if the 1^st and 4^th numbers can't be 0?

• A. 10
• B. 90
• C. 8,100,000,000
• D. 10,000,000,000

Answer: (C)

Verified step by step guidance

1. Understand that a phone number is a sequence of 10 digits, where each digit can range from 0 to 9.
2. Identify the constraints: the 1st and 4th digits cannot be 0, meaning they can be any digit from 1 to 9.
3. Calculate the number of choices for the 1st digit: since it cannot be 0, there are 9 possible choices (1-9).
4. Calculate the number of choices for the 4th digit: similarly, there are 9 possible choices (1-9).
5. Calculate the number of choices for the remaining 8 digits: each can be any digit from 0 to 9, giving 10 choices per digit. Multiply the choices for each digit to find the total number of possible phone numbers.