Among all monthly bills from a certain credit card company, the mean amount billed was $485 and the standard deviation was $300. In addition, for 15% of the bills, the amount billed was greater than...

Among all monthly bills from a certain credit card company, the mean amount billed was $485 and the standard deviation was $300. In addition, for 15% of the bills, the amount billed was greater than $1000. A sample of 900 bills is drawn. a) What is the probability that the average amount billed on the sample bills is greater than $500? b) What is the probability that more than 150 of the sampled bills are for amounts greater than $1000?