What is the greatest common factor of all two-digit palindromes?  (Note: A palindrome is a number that reads the same forwards as backwards.)
I know that the two-digit palindromes are 11, 22, 33, 44, 55, 66, 77, 88, and 99. 11 is already prime, so I can't factor it further. Therefore, the greatest common factor of all two-digit palindromes must be 11, since it is the largest factor that they all share. 