If population growth rate is 1.2-1.5%, population doubles in how many years?

ANSWER: (D) 47-58 yearsREF: Park 20th ed p. 412, Brief Principles of Macroeconomics by N. Gregory Mankiw 6th ed page 180Answer to this question lies in the table provided in previous question- Surely it is difficult to remember the above mentioned table, but there is always a better way.Rule of 70:An old rule of thumb, called the rule of 70, is helpful in understanding growth rates and the effects of compounding. According to the rule of 70, if some variable grows at a rate of x% per year, then that variable doubles in approximately70/x years.Doubling time = 70/Growth rateAll the values in the table of previous question can be calculated using the same formula. So for growth rate between 1.2 to 1.5% the doubling time wil be 47 to 58 years