5. [7 marks] More hitting 10. Suppose you have a biased coin that comes up heads with probability s and tails with probability 1-s. If you toss the coin repeatedly, with heads worth 2 and tails worth 1, and add up the score as you go along, what is the probability pn that you'll hit n? Check your formula in two ways, first with a direct calculation of p2, and secondly by comparing with the s = 21 case that we solved in class.