2026 AIME I Problem 2

Attempt Problem 2 of the 2026 AIME I below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2026 AIME I solutions, or check the answer key.

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2.

Find the number of positive integer palindromes written in base 10,10, with no zero digits, and whose digits add up to 13.13. For example, 4212442124 has these properties. Recall that a palindrome is a number whose representation reads the same from left to right as from right to left.

Answer: 62
Concepts:palindromedigitspartitions and compositionsparity
Difficulty rating: 2110
Solution:

A palindrome with an even number of digits has each digit appearing in a mirrored pair, so its digit sum is even. Since 1313 is odd, the palindrome has an odd number of digits, and if mm is the middle digit, the rest of the digit sum 13m13 - m is split evenly between the two halves, so mm is odd. A one-digit palindrome would need m=13,m = 13, which is impossible.

The palindrome is determined by its middle digit mm and the block of digits to the left of center: a nonempty string of nonzero digits with sum s=13m2.s = \frac{13 - m}{2}. For m=1,3,5,7,9m = 1, 3, 5, 7, 9 we get s=6,5,4,3,2.s = 6, 5, 4, 3, 2. Since s6,s \le 6, every digit of such a string is automatically at most 9,9, so the number of strings is the number of compositions of s,s, which is 2s12^{s-1} (each of the s1s - 1 gaps between units is either a break or not).

The total is 25+24+23+22+21=32+16+8+4+2=62. \begin{aligned} &2^{5} + 2^{4} + 2^{3} + 2^{2} + 2^{1} \\ &= 32 + 16 + 8 + 4 + 2 \\ &= 62. \end{aligned}

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