A box contains 15 red discs and 5 black discs. A disc is selected at random and its color noted. The disc is then replaced in the box.
(a) In eight such selections, what is the probability that a black disc is selected
(i) exactly once?
(ii) at least once?
(b) The process of selecting and replacing is carried out 400 times.
What is the expected number of black discs that would be drawn?