Let's start with rabbits
Note: We count this in pairs. So 1 rabbit is actually one pair of rabbits. ( edit: always one male and one female.)
We start with 1 young rabbit. Given that:
Young rabbits can reproduce after two months.
They will give birth to a rabbit every month from there.
They do not die.
How many rabbits will there be in 1 year after the first young rabbit is placed?
Note that this question is very vague/unrealistic. Rabbits do die, not all are of different gender, and they do not give birth that periodically.