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Printjmc
number theory intermediate
Problem
How many positive four-digit integers are divisible by ?
Solution
An integer is divisible by if and only if the number formed from its last three digits is divisible by . Thus, the number of possibilities for the last three digits is equal to the number of three-digit multiples of . Since , we find that there are such multiples. Since the thousands digit of our four-digit integer must be nonzero, there are possibilities for the thousands digit. Altogether, four-digit integers are divisible by .
Final answer
1125