In a game of Archery, each ring of the Archery target is valued. The centremost ring is worth 10 points and rest of the rings are allotted points 9 to 1 in sequential order moving outwards. Archer A is likely to earn 10 points with a probability of 0.8 and Archer B is likely the earn 10 points with a probability of 0.9. Based on the above information, answer the following questions : If both of them hit the Archery target, then find the probability that (a) exactly one of them earns 10 points. (b) both of them earn 10 points.