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jmc

counting and probability senior

Problem

How many positive, three-digit integers contain at least one as a digit but do not contain a as a digit?
Solution
Let us consider the number of three-digit integers that do not contain and as digits; let this set be . For any such number, there would be possible choices for the hundreds digit (excluding , and ), and possible choices for each of the tens and ones digits. Thus, there are three-digit integers without a or .

Now, we count the number of three-digit integers that just do not contain a as a digit; let this set be . There would be possible choices for the hundreds digit, and for each of the others, giving . By the complementary principle, the set of three-digit integers with at least one and no s is the number of integers in but not . There are such numbers.
Final answer
200