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?