In this simple alternating subtraction and addition series; 1 is subtracted, then 2 is added, and so on. Rule = (first number – 1) (second number + 2) (third number – 1) (fourth number + 2)…… Now The series is 22, 21, 23, 22, 24, 23, . . . (22 – 1 = 21), (21 + 2 = 23), (23 – 1 = 22), (22 + 2 = 24), (24 – 1 = 23) (23 + 2 = 25)… So the answer is c = 25.