Help please, just can't work this one out!

[ljc7]

This is in one of the Athey Educational VR papers.

AE is to IO as BC is to ?

DS usually storms through these, but we are both stuck

MC answers DE, FG, MN, ME, DF, and GD

Answer is DF.

Applying usual methods we get JM

Any input very gratefully received!!

Many thanks

Lucy

[aliportico]

Is it vowels and consonants?

AE then IO (U)

BC then DF (GHJKl etc)

Very difficult if so!

[ljc7]

Hadn't considered that!

Many thanks, will try and post the result