Given this stated question
I don't believe you have your equation worded correctly. It should be P(A|B) = probability of A (both children are girls) given probability of B (at least one of them is a girl), which when the correct numbers are plugged in becomes
P(A|B) = P(A ∩ B)/ P(B) = ¼ / ¾ = 1/3
Same equation, but the set-up is key. I'm sure you would agree that there a 1/4 chance that both children are girls (GG out of GG,BG,GB,BB) and a 3/4 chance that there is at least one girl (GG,BG,GB). So the correct and only answer must be 1/3. QED
I hope my html character entities show up properly. Prime would be proud.