The factor of every palindrome with an even number of digits?

Answer 1

Every palindrome with an even number of digits is divisible by 11. The easiest way to see this is to recall the divisibility rule by 11: if a number X is written as ABCDEFG... (here A,B,C, ... are digits), then it's divisible by 11 if and only if the sum A-B+C-D+E-F+G-... is divisible by 11. In a palindrome with an even number of digits, each digit will appear in an odd position and in an even position, so when we calculate this sum, it will be added once and subtracted once, canceling. Since all the digits cancel, the sum A-B+C-D+... will be 0, which is divisible by 11. So the original number ABCD....DCBA was also divisible by 11.