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 Question from Jacob, a student: If the last 4 digits of a cell phone number, XXX-XXX-ABCD, are restricted where: A cannot be 0 or 1 and B cannot be 0 / how many possible 4 digit combinations are left?

Hi Jacob,

Let's start from the right end. There are $10$ choices for D and for each of these choices there are $10$ choices for C. Thus there are $10 \times 10 = 100$ choices for the two digit number CD. How many choices are there for B? How many for the three digit number BCD? How many choices for A? How many for ABCD?

Penny

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