In this series, each number is repeated, then 13 is subtracted to arrive at the next number.
Rule = (1st number) (1st number) (1st number - 13 = 2nd number) (2nd number repeat) (2nd - 13 = 3rd number) (3rd number repeat)....
The series is 53, 53, 40, 40, 27, 27...
(53),(53 - 13 = 40), (40), (40 - 13 = 27) (27), (27 - 13 = 14), (14)...
So the answer is b = 14.