boomgoesthegmat wrote:At the start of an experiment, a certain population consisted of 3 animals. At the end of each month after the start of the experiment, the population size was double its size at the beginning of that month. Which of the following represents the population size at the end of 10 months?

A) 2^3
B) 3^2
C) 2(3^10)
D) 3(2^10)
E) 3(10^2)

This is a great candidate for creating a growth table (for more on this see this video: https://www.gmatprepnow.com/module/gmat ... /video/929 )

- Start: 3 animals
- End of month 1: (3)(2) animals [since the population doubles each month]
- End of month 2: (3)(2)(2) animals
- End of month 3: (3)(2)(2)(2) animals
- End of month 4: (3)(2)(2)(2)(2) animals
.
.
.
We can see the pattern and recognize that, at the end of month 10, we'll have the product of one 3 and ten 2's.
.
.
- End of month 10: (3)(2)^10

Answer: D

Cheers,
Brent

Hi All,

This is a great 'concept' question - if you recognize the concept involved, then you don't have to do any actual math to solve this problem. If you're not sure how the math 'works' however, you can still use a bit of 'brute force' to define the pattern involved and still get the correct answer.

We're told that a population starts with 3 animals and at the end of each month, the population DOUBLES. We're asked for the size of the population at the end of 10 months.

Based on the 'growth' that's defined, we can list out the first few months worth of data for reference....

Start = 3 animals
Mo. 1 = 3(2) = 6 animals
Mo. 2 = 6(2) = 12 animals
Mo. 3 = 12(2) = 24 animals
Etc.

Notice how each month we multiply the prior month by TWO. That is the pattern involved. We start with 3, then multiply by 2 over-and-over. Since there are 10 months, there will be 10 "multiply by 2s", so the correct answer must be....

Final Answer: D

GMAT assassins aren't born, they're made,
Rich