Maths Olympiad Prep

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Combinatorics Difficulty 3.1 AMC 10/12 Find the answer

How many four-digit positive integers have at least one digit that is a 22 or a 33?

Pick one

Solution

Since we are asked for the number of positive 44-digit integers with at least 22 or 33 in it, we can find this by finding the total number of 44-digit integers and subtracting off those which do not have any 22s or 33s as digits.
The total number of 44-digit integers is 9101010=90009 \cdot 10 \cdot 10 \cdot 10 = 9000, since we have 1010 choices for each digit except the first (which can't be 00).
Similarly, the total number of 44-digit integers without any 22 or 33 is 7888=35847 \cdot 8 \cdot 8 \cdot 8 ={3584}.
Therefore, the total number of positive 44-digit integers that have at least one 22 or 33 is 90003584=(E) 5416.9000-3584=\boxed{\textbf{(E) }5416}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.