a) 13000 divided by 2 is 6500, which was the population 10 years ago. 6500 divided by 2 is 3250, which was the population 20 years ago. 3250 divided by 2 is 1625, which was the bird population 30 years ago. 25 is halfway between 20 and 30, and 2437.5 is halfway between 3250 and 1625, so I would say that 2437.5 is the answer.
b) Well 13000 times 2 gives you 26000, which would be in ten ears, and halfway between 13000 and 26000 is 19500, which gives you the bird population in five years. :-)