A certain species of bird was introduced in a certain country 25yrs ago. Biologists observe that the population doubles every 10 years, and now the population is 13,000.

a. What was the initial size of the bird population?

b. Estimate the bird population 5 years from now.

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. : - )