Question 878785
Telephone numbers in the US start with a 3 digit area code followed by a 7 digit local number.  Assume that any number (0-9) is possible for each digit.

a)	How many different area codes are possible?
3 decimal digits.  What do you think?
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b)	What is the probability that an area code starts with a 5?
5 is 1 of 10 possibilities.
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c)	For a given area code, how many different local numbers are possible?
7 decimal digits.
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d)	How many different complete telephone numbers are possible?
10 decimal digits --> {{{10^10}}}