A phone number is an ordered string of ten digits. Each digit is
one of 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9.
(a) How many phone numbers are there that do not contain any
(b) How many phone numbers are there that start with 216 or
(c) How many phone numbers are there that start or end with
(d) What is the smallest number of phone numbers that guarantee
at least nine of them start with the same two digits?
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