The pattern in this series involves alternating between adding 4 and subtracting 3. Starting with 72, we add 4 to get 76, subtract 3 to get 73, add 4 to get 77, subtract 3 to get 74, and so on. Therefore, the number that should fill the blank in the series is obtained by adding 4 to the previous term, resulting in 75. Therefore, the number that should fill the blank is 75.
