For a value that is growing exponentially (population, in this case), doubling time is the amount of time in which the value doubles. In your case, population doubles every 30 years, so in 120 years it doubles four times. That results in a growth by factor 2⁴.
Answers & Comments
2^4 = 16
2^(120/30) =>
2^4 =>
16
It's the amount of time between a doubling of the population.
For example, let's say we start with 100,000 people.
After 30 years, we'd expect that to double to 200,000 people.
After 30 more years (60 total), we'd expect that to double to 400,000 people
After 30 more years (90 total), we'd expect that to double to 800,000 people
After 30 more years (120 total), we'd expect that to double to 1,600,000 people
But we can figure this out in one step rather than having to break it down into 4 steps.
120 years would be *4* doubling periods (of 30 years).
2^4 = 16
So it means the population would be 16 times as much.
2 * 2 * 2 * 2 = 2^4 = 16 times
For a value that is growing exponentially (population, in this case), doubling time is the amount of time in which the value doubles. In your case, population doubles every 30 years, so in 120 years it doubles four times. That results in a growth by factor 2⁴.
2^(120/30) =>
2^4 =>
16
30 years = x2 of now
60 years = x2 of ^^ above
120 years = x2 ^^ above.