3 — there will be the two rabbits from the previous month, plus one new-born.
| Month | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |
| Female Rabbits | 1 | 1 | 2 | 3 | ||||||
At the start of month 4 there is one mature rabbit, one immature rabbit, and one new-born rabbit. During that month, the immature rabbit becomes mature, so at the start of month 5 there will be two mature rabbits. During month 5 both mature rabbits give birth.
Rabbits become mature during their second month, so every rabbit listed for month 3 will be mature in month 5 and give birth to one rabbit in month 5 (and every month following).
In the picture, every rabbit is labeled with her month of birth. A rabbit will start giving birth in the month of her birth plus two. So the rabbit labeled "1" will give birth in months 3, 4, 5, ...
The rabbit labeled "3" will give birth in months 5, 6, 7, ...
So the number of rabbits born in month 5 is the number of rabbits listed under month 3.
Add the number of rabbits for month 4 to the number of rabbits born in month 5 to get the total number of rabbits for month 5.
How many rabbits are there during month 5?