Sender: "Hamilton Weston"
Subject: Phone Number Possibilities

How do phone companies arrive at the possible number of phone numbers that can be generated for customers?

Hi Hamilton,

If you want to count the number of possible 7 digit "phone numbers" and you are allowed any of the 10 digits in any position then there are 10x10x10x10x10x10x10 = 107 = 10 000 000 possibilities. But this is not the way the system works. The last 4 digits are not constrained (although I have never seen xxx-0000 as a phone number) but the first three digits specify the exchange and I don't know the restrictions on exchanges. So the best I can say is that there are 10 000 (or maybe 9 999) phone numbers per exchange.


Go to Math Central