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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..

 Oct 6, 2020
edited by Guest  Oct 6, 2020
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By Inclusion-Exclusion, the number of telephone numbers I can remember is 1844.

 Oct 6, 2020

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