The number of days between billing and payment of charge accounts at Groshen's, a large department store, is approximately normally distributed with a mean of 18 days and standard deviation of 4 days.
a. What is the probability that a bill will be paid between 12 and 18 days?
b. What is the probability that a bill will be paid between 20 and 23 days?
c. What is probability that a bill will be paid between 10 and 20 days?
d. What is the probability that a bill will take longer than 21 days to be paid?
e. What is the probability that a bill will take less than 16 days to be paid?