Probability help (this is driving me nuts)

I have been at this for 3 days already and i did some of them but i just am so confused that my brain hurts even trying to figure out if i am doing the right stuff. please please help me/ show me or point me in the right direction with these problems

5. There is a 50-50 chance that Barbara carries the gene for hemophilia. If she is a carrier,

then each of her children has a 50-50 chance of having hemophilia. Suppose that Barbara has had three

children, and none of them have the disease. What is the probability that Barbara is a carrier? If she has a

fourth child, what is the probability that the child will have hemophilia?

6. In a class there are 4 freshman boys, 6 freshman girls, and 6 sophomore boys. How many

sophomore girls must be present if sex and class are to be independent when a student is selected at random?