Question

Suppose a population of 250 crickets doubles in size every 3 months. How many crickets will there be after 2 years?

Answer 1

Start by writing what is occurring, as see if you can find a formula based on that
250 crickets -> 3 months
500 crickets -> 6 months
1000 crickets -> 9 months
2000 crickets -> 12 months
...

Or another way is:
250 -> 3 months
2(250) -> 6 months
2[2(250)] -> 9 months
2{2[2(250)]} -> 12 months
...

Answer 2

Notice that you keep multiplying 2's :)

Answer 3

You don't necessarily need to find a formula though, you can just keep continuing the pattern until 24 months (2 years)