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2013 AMC 12A Problem 22

Problem 22 of 25HarderNumber TheoryCounting & Probability

A palindrome is a nonnegative integer number that reads the same forwards and backwards when written in base 1010 with no leading zeros. A 66-digit palindrome nn is chosen uniformly at random. What is the probability that n11\dfrac{n}{11} is also a palindrome?

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Solution

Let m=n11.m= \frac{n}{11}. If mm had four digits, then n<110000,n<110000, so the first and last digits of the six-digit palindrome nn would both be 1.1. This forces the first and last digits of the palindromic mm to be 1,1, hence m<2000m<2000 and n<22000,n<22000, a contradiction. Therefore mm is a five-digit palindrome abcba.\overline{abcba}. Writing n=11m=abcba0+abcba,n=11m=\overline{abcba0}+\overline{abcba}, no carries occur exactly when a+b9a+b\le9 and b+c9;b+c\le9; the resulting digits are a,a+b,b+c,b+c,a+b,a.a,a+b,b+c,b+c,a+b,a. If a+b10,a+b\ge10, the leading and trailing digits differ; if only b+c10,b+c\ge10, the next pair differs. Thus the conditions are also necessary. The number of valid mm is b=09(10b)(9b)=330. \sum_{b=0}^{9}(10 - b)(9 - b) = 330. There are 9102=9009\cdot 10^2 = 900 six-digit palindromes, so the probability is 330900=1130.\dfrac{330}{900} = \dfrac{11}{30}. Thus, the correct answer is E.

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Concepts: palindrome · basic probability · digits

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