Joonkim - It's a sex linked recessive lethal gene. The fact that it is sex linked means that it is carried on the X chromosome. It is impossible for a male to have it and be living since he only has one X chromosome. Make sense?

Anyway, I don't like either of your approaches. Nze I think if you do it that way you're making an assumption that only 1 male has it. There's a 50% chance that a male will get it, so out of 3 boys, 2 could have it also.

The way I did it was this:

We already know that the mother is XaX, and the father must be XY (as described above). Each time they have children, it will come out as XaX, XX, XaY, XY. That means the XaY male will die, leaving 3 kids, 1 of them being a boy.

When she has 3 more kids, you have the same scenario. 1 boy + 1 boy = 2 boys total.