Pattern in Alternate/Adjacent numbers:This is a complex series in which the successive letters decrease or multiplied by ... Thus, the number in the blank will be
If we go in reverse order alphabetically in first character in the series . We get the series of alphabets B,C,D,E,F. Therefore the first character of missing alphanumeric is definitely E. Again in tracing the series in reverse order an dividing the numerical part each time by 2 we get the series: 32,16,8,4,2. Therefore the second character of missing alphanumeric is definitely 4.
By having this both combinations, its pretty clear that it is E4.