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Combinatorics

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Phone Numbers

COMBIN-GJ156L

Telephone numbers consist of a $3$-digit area code, followed by a $3$-digit prefix and a $4$-digit extension. Within the area code $740$ the telephone company has assigned the prefix $427$ as a prefix for a certain city.

What is the largest number of customers the telephone company can serve in this city before being forced to introduce a new prefix?

A

$40$

B

$1000$

C

$9000$

D

$10000$

E

$1000000$