Four digit phone numbers

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Question from Tom, a teacher:

Hi, I'm an online math teacher, working on lessons for my students.

This question is in their book and I can't do it...! Help!

It has to do with phone numbers, and in this problem,
we are only dealing with
the last 4 digits of the number.

It asks how many possible numbers we have
if at least one digit repeats in the last 4 digits of the number.

Hi Tom,

Suppose you had a list of all 4 digit phone numbers with no restrictions and you removed all the numbers with no digit repeated. What would you have left?

Write back if you need more assistance,
Penny

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