I am having trouble with Q3 where we need to express the distribution P(x1 | x1 + ... + xn)
If I let this equal ( P(x1 = a) * P(x2 + ... + xn = N-a) )/ P(x1 + ... + xn = N), I almost get a binomial distribution like:
(N choose a) * p^a * q^N-a
but it seems that I'm not getting a probability for p and q but instead I have n, the number of random variables

Anyone know what I'm doing wrong?