4. Probability
Fundamental Counting Principle
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Phone numbers are 10 digits long. How many possible phone numbers are there if the 1st and 4th numbers can't be 0?
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Verified step by step guidance1
Understand that a phone number is a sequence of 10 digits, where each digit can range from 0 to 9.
Identify the constraints: the 1st and 4th digits cannot be 0. This means for these positions, the digits can range from 1 to 9.
Calculate the number of possibilities for the 1st digit. Since it cannot be 0, there are 9 possible choices (1 through 9).
Calculate the number of possibilities for the 4th digit. Similarly, since it cannot be 0, there are 9 possible choices (1 through 9).
Calculate the number of possibilities for the remaining 8 digits (2nd, 3rd, 5th, 6th, 7th, 8th, 9th, and 10th). Each of these can be any digit from 0 to 9, giving 10 possible choices for each. Multiply the possibilities for each digit position to find the total number of possible phone numbers.
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