(1.54=6 marks) How many bit strings of length 12 contain (a) exactly three 1s? (b) at most three 1s? (c) at least three 1s? (d) an equal number of 0 s and 1s? (3 marks) Suppose that a bag contains six slips of paper: one with the number written on it, two with the number 2 , and three with the number 3 . What is he expected value of the number drawn if one slip is selected at random from the ag