2 more then im done so help this girl

Little’s law firm has just one lawyer. customers arrive randomly at an average rate of 6 per 8 hour workday. service times have a mean of 50 minutes and a standard deviation of 20 minutes. how long does a customer spend at little’s law firm on average?

Solve this problem! extra ! 3025/5.5 = a/90.75

Prove that 16^4-2^13-4^5 is divisible by 11.