(10 points) The average cost of repairing a phone is $120 with a standard deviation of $10.50. (a) (3p) If 15% of the costs are considered excessive, find the cost in dollars that would be considered excessive. (b) (3p) If there is a phone being repaired, find the probability that the cost of the phone will be at least $115.5. (c) (4p) If a sample of 30 phones are selected to be repaired, find the probability that the sample mean of the cost will be at least $115.5