4. The pH levels of food items prepared in a certain viva}.r are normally distributed with a standard deviation of a = 0.72. An experimenter estimates the mean pH level by averaging the pH levels of a random sample of 71 items. a) If n = 7, what is the probability that the experimenter's estimate is within 0.5 of the true mean value? b) What sample size 11 is needed to ensure that there is a probability of at least 95% that the experimenter's estimate is within 0.5 of the true mean value