Seven-digit telephone numbers are not allowed to begin with 0 or 1.

I can only remember a seven-digit telephone number if the first three digits (the "prefix") are equal to the next three digits or the last three digits. For example, I can remember 389-3892 and 274-9274.

How many seven-digit telephone numbers can I remember?

Please answer with detailed explanation..

Guest Oct 6, 2020

edited by
Guest
Oct 6, 2020