Final answer:
The world's population is estimated to be approximately 48 billion people in 2099, assuming it doubles every 45 years.
Step-by-step explanation:
To estimate the world's population in 2099, we need to determine how many times it will double between 1999 and 2099. Given that it doubles every 45 years, we can calculate the number of doubling periods by dividing the time span by 45: (2099 - 1999) / 45 = 2.22222. Since it's not possible to have a fraction of a doubling period, we round up to the nearest whole number, which is 3.
Now, we can double the initial population of 6 billion three times: 6 billion x 2 x 2 x 2 = 48 billion.
Therefore, the estimated world's population in 2099, assuming it doubles every 45 years, is approximately 48 billion people.