Probability that last child is a boy Johnny has 4 children. It is known that he has more daughters
than sons. Find the probability that the last child is a boy.

I let A be the event that the last child is a boy, P(A) = $\frac{1}{2}$.
and B be the event that he as more daughters than sons. But im not sure how to calculate P(B) and what are the subsequent steps to take after.

Appreciate any help.

Thanks
If he has more daughters than sons, Below are the 5 possible cases:   

D D D D --> All Daughters
S D D D --> 3 Daughters
D S D D
D D S D
D D D S    

So probability of having last child as son is = 1/5.
They are mutually exclusive events. How many son/daughters the couple already has is irrelevant. Hence, the probability of the last child (or rather any child) being boy is 1/2 (some would say 1/3 too).