Wye-delta transform derivation
Unfortunally, as I just created my account here, I cannot comment. So this is not a complete answer.

The way you need to look at it, is that the equivalent resistance from nodes a-b, a-c an b-c must be equal comparing between both designs, so that's where you start your math.

If you want to, you can start algebra from that and you will get the answer.

I'll edit this answer ASAP in order to give you the full explanation including algebra stuff, I'm at work now :)

Hope it helps.