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 with no leading zeros. A -digit palindrome is chosen uniformly at random. What is the probability that is also a palindrome?
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Solution
Let If had four digits, then so the first and last digits of the six-digit palindrome would both be This forces the first and last digits of the palindromic to be hence and a contradiction. Therefore is a five-digit palindrome
Writing no carries occur exactly when and the resulting digits are If the leading and trailing digits differ; if only the next pair differs. Thus the conditions are also necessary. The number of valid is
There are six-digit palindromes, so the probability is
Thus, the correct answer is E.