EFG is related to 567 in the same way as OPQ is related to:

The relationship represents the positional values of the English letters in alphabetical order ($A=1,B=2,\dots ,Z=26$):
* E = 5
* F = 6
* G = 7
Combining them gives 567.

Applying the same logic to OPQ:

* O = 15
* P = 16
* Q = 17
Combining them gives 151617.