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
The given series is a combination of alphabets and numbers.
The alphabets proceed in the backward manner. So, after F, it will be E.
The numbers proceed in the powers of 2, therefore the number after 2 will be a square of 2 i.e. 4.
Therefore the number in the blank will be E4.