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A phone number is an ordered string of ten digits. Each digit is one of 0,...

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 zeros?

(b) How many phone numbers are there that start with 216 or 440?

(c) How many phone numbers are there that start or end with 3?

(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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Answer #1

So total phone no lo - = 9+ Lot lo t e lo If any phone no. may start from a then total phone nous = lof lofa = 100

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