On The Average, A Certain Light Bulb Is Supposed To Last 450 Hours. If It Takes Mary An Average Of 6 1/4 Hours To Read A Novel, What Is The Maximum Number Of Novels Mary Can Expect To Finish Reading During The Life Of The Bulb?

A number of assumptions are involved regarding operating environment, utilization fraction, and probability of failure as a function of time. The precise meanings of the words "average", "supposed to last", "maximum", and "expect" are involved. The details are too arcane to go into here.
  (450 hours)/(6 1/4 hours/novel)
  = (450/(25/4)) novels
  = (450*4/25) novels
  = 72 novels