Find the missing pair in the series, 

EQ, FS, HV, KZ, _________, TK.

OE

First letters:

E → F → H → K → O → T

Positions:

• E = 5
• F = 6 (+1)
• H = 8 (+2)
• K = 11 (+3)
• O = 15 (+4)
• T = 20 (+5)

So the missing first letter is O.

Second letters:

Q → S → V → Z → E → K

Positions:

• Q = 17
• S = 19 (+2)
• V = 22 (+3)
• Z = 26 (+4)
• E = 5 (after Z, +5 with wrap-around)
• K = 11 (+6)

So the missing second letter is E.

Missing pair = OE