108 , 54 , 36 ,18 , 9 , 6 , 4

The given series is in the below pattern,we can see that
Next term = Previous term ÷ n { putting n = 2 and 1.5 alternately }
First term = 108
Second term = First term ÷ 2
? Second term = 108 ÷ 2 = 54 ;
Third term = Second term ÷ 1.5
? Third term = 54 ÷ 1.5 = 36 ;
Third term = Third term ÷ 2
? Fourth term = 36 ÷ 2 = 18 ;
Fifth term = Fourth term ÷ 1.5
? Fifth term = 18 ÷ 1.5 = 9 is wrong. It should be 12.
Sixth term = Fifth term ÷ 2
? Sixth term = 12 ÷ 2 = 6 ;
Seventh term = Sixth term ÷ 1.5
? Seventh term = 8 ÷ 1.5 = 4
Hence, 9 is wrong. It should be 12.