Explanation:

As per the given question,
104 , 108 , 54, 58 , 29, 31
Series is following the given pattern,
Series going on adding 4 then divide by 2,
104 +4 =108, 108 /2 = 54, 54 +4 = 58 , 58 /2 = 29 , 29 +4 = 33.
then given series 31 is wrong right is 33.
Ans is 31