Average yearly inflation. if an item costs c at one time and d n years later, and cxn = d, then we call x the average annual inflation factor (for example, x = 1.04 refers to an inflation rate of 4%). at a 6% average annual inflation factor, it will take 12 years for the house to double. if 6% is replaced by r%, bankers use 72/r to estimate the number of years. this is called the rule of 72. graph this estimate and the actual answer over the interval 1 < = r < = 12. comment on the accuracy of the rule of 72.