the doubling time of a city's population is 13 years. how long does it take for the population to quadruple.

Lets say the population is 10. It takes 13 years to double to 20. Once it is at 20, it takes another 13 years to double again and be at 40. It takes 26 years for a city's population to quadruple because it will double in 13 years, and then that population will double and take another 13 years.