Question 145211
Is too hard to draw the tree here. That video has a good one in it.

As far as probability goes
there is 1/4 chance the first 2 are both girls. Thus there is a 3/4 chance the first two are not both girls.

So we have 1/4 + (3/4)*(1/2) = 1/4 + 3/8 = 1/2

If you draw the tree and count those branches with at least 2 girls on it, you'll see there are four of them. Since there are 8 possible outcomes, that is also a way to get to 4/8 = 50%