The amounts of time per workout an athlete uses a stairclimber are normally distributed, with a mean of 21 minutes and a standard deviation of 7 minutes. Find the probability that a randomly selected athlete uses a stairclimber for (a) less than 18 minutes, (b) between 21 and 29 minutes, and (c) more than 30 minutes. (a) The probability that a randomly selected athlete uses a stairclimber for less than 18 minutes is (Round to four decimal places as needed.) (b) The probability that a randomly selected athlete uses a stairclimber between 21 and 29 minutes is (Round to four decimal places as needed) (c) The probability that a randomly selected athlete uses a stairclimber for more than 30 minutes is (Round to four decimal places as needed.)