for one group of middle income customers (you call this group B1), you have studied historical data and believe that the variable "amount of credit card interest ($) in a month" has a normal distribution with a mean of $15 and a standard Deviation of $2.5. What is the long run average interest/month? what is the stdev of interest/month? what is the estimated proportion who pay > $20 interest/month?what is the estimated proportion who pay <=$10 interest/month? what is the interest that is exceeded by only 10% of the group?