how many numbers s should be chosen from a set S $=\backslash 1,...,$n$\backslash$ such that there exists $(3$ marks$)$a pair of numbers whose sum isn$/28.$ A bit string consisting of $0\text{'}$s and I$\text{'}$s is a palindrome if its reversal is same as the original$(3$ marks$)$ string. For instance, $000111$ is not a palindrome where as $1010101$ and $101101$ are palindromes. How many bit strings of length n are palindromes.