Question 4 A. The number of masks sold per day at a retail store is shown in the table below, with the corresponding probabilities. Find the average of the distribution. If the owner of the retail store wants to be sure that he has enough masks for the next 7 days, how many should the owner purchase? Number of masks sold X 18 19 20 21 22 Probability P(X) 0.1 0.2 0.3 0.3 0.1 B. The average bus fare of a student to travel to USP daily is $5.31. If the distribution of bus fares is approximately normal with a standard deviation of $0.31, what is the probability that a randomly selected bus fare is less than $4.50? C. The average monthly salary of staffs at USP is $4164 in a recent year. If the salaries are normally distributed with a standard deviation of $360, find the probability that the mean salary for a random sample of 20 staffs is less than $3900.