The way you could work this out is by trial and error. 11 % a year means that its population increases by 4400 annually. 40,000 X 1.11^1 gives you the amount it would be after one year (44000). I got the 1.11 because of the percentage (0.11) plus the amount you need to do percentage increase (1). Then all you do is keep on going up till you hit the number you need (80,000) after about 7 years the total would have hit 80,000 and would have actually went past it (83046)
