Answer:
4/12 or 1/3
Step-by-step explanation:

function g has to be the transformation of function f. Therefore,

is the answer.
Assuming that the probability of boys and girls is equal (in reality slightly more boys than girls are born), the probability is 1/2, simply because the older girl's gender is given and therefore not a part of the probability.
This question comes from the 'boy or girl paradox', a.k.a. the Mrs Smith Problem.