A string is called a palindrome if it reads the same when reversed. That is, the string x1 x2 ... xn is a palindrome if x1 x2 ... xn = xn ... x2 x1. The bit strings 11011 and XXXXXXXXXXare...

A string is called apalindrome if it reads the same when reversed. That is, the string x1 x2 ... xn is a palindrome if x1 x2 ... xn = xn ... x2 x1.

The bit strings 11011 and 01011010 are palindromes.
The bit strings 1011 and 01111 are not.

How many bit-strings of length 7 are palindromes?