posted by Sam .
The doubling time of a city's population is 17 years. How long does it take for the population to quadruple?
as long as it takes to double twice: 34 years
How did you get that?
come on, man. quadrupling means doubling twice. Each doubling takes 17 years.
after 17 years, the initial population P has grown to 2P
after 17 more years, the population 2P has grown to 2(2P) = 4P, and is quadruple the initial population of P.