I have a question
If I have a certain card, with for example 10 digits. Do you think that is it possible that someone else have the same last 4 digits of my card?
I think that is possible. Because if the number has 10 digits I only care about the first six digits. that can be any number between 0 and 9, so there are $10^6$ possibilities to create another card with the same last 4 digits as mine.
Am I thinking correctly? Can anyone help me?
Thanks
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$\begingroup$Your thinking is correct. If there are more than a million such cards, it is certain that two of them share the same last four digits. It is still possible (but unlikely) that your last four digits are unique. Maybe there are $10^7$ cards out there with all possibilities for the first seven digits represented and all three of the last being $000$. If your last three are not $000$, there are no duplicates for you. Assuming the numbers are unique, there are $10^6-1$ other numbers that would match yours in the last four digits.
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