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jmc

number theory intermediate

Problem

Find the smallest three-digit palindrome whose product with 101 is not a five-digit palindrome.
Solution
We can use the distributive property of multiplication to multiply a three-digit palindrome (where and are digits) with 101: Here, the digits of the product are , , , , and , unless carrying occurs. In fact, this product is a palindrome unless carrying occurs, and that could only happen when . Since we want the smallest such palindrome in which carrying occurs, we want the smallest possible value of such that and the smallest possible value of . This gives us as our answer and we see that is not a palindrome.
Final answer
505