35 is indeed the correct answer.
The key to this puzzle is looking at different numerical bases. Were it constructed in binary or hexidecimal, that would have been a dead giveaway. Instead, I chose octal, and started with two integer primes, where 11=13, 13=15, 17=21, 19=23, 23=27, and 29=35.
I was all but certain you folks had it cracked with the 2,4 logic postings starting on the 16th.
An eight-legged fuzzy bunny to
fernweh