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jmc

number theory senior

Problem

Suppose that a -digit integer is composed of thirteen s and seventeen s. What is the remainder when is divided by ?
Solution
By the Chinese Remainder Theorem, it suffices to find the remainders when is divided by and . The last two digits of must be one of or ; each of these leave a remainder of after division by . By the divisibility property of , it follows that .

The sum of the digits of is equal to . This leaves a remainder of after division by , so it follows that .

By the Chinese Remainder Theorem and inspection, it follows that satisfies the two congruences, and so leaves a remainder of upon division by .
Final answer
25