Umm I didn't get a domain.... No solutions were found...
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.
You must multiply 5x-3 with 5, then you will get
25x-15 = 1
25x = 1+15
25x = 16

x = 0.64
16,330 is the nearest ten