Question

If the annual inflation rate is 5 percent a year, about how many years will it take for the price level to double?

Answer 1

According to the rule of 72, the doubling time for price level when the inflation rate is r%, is given by
$t= \frac{ln(2)}{ln(1+r/100)} \approx \frac{72}{r}$

Therefore
t = 72/5 = 14.4 years

Answer: About 14 years.