By definition W is a binomial random variable so it's distribution (PMF) can be represented by:
P(W = m) = ((n-k) over m) where m is the number of questions missed.
P(W = m) = (4/5)^m *(1 - 4/5)^(n-k-m)
B) C = n - W
P(C) = k + ((n-k) over r) (1/5)^r * (1 - 1/5)^(n-k-r)
