From onward, the number of daily visitors to a website increased by every two years. So, for example, the number of visitors

Well this question is very odd if you think about it, since most people would say 2011 because it seems obvious. They would be wrong! This isn't the same as doubleing because you are not doing 200% + 200%, but instead it will be 200% * 400% so the answer to this problem is 2007 using our equation. Could I get confirmation?

https://web2.0calc.com/questions/from-2003-onward-the-number-of-daily-visitors-to  this was a question from a while back. It might have a better explanation!

Date            Increase          number of visitors            

2003                0                          N                           

2005                2N               N+2N = 3N                    

2007             2*3N = 6N      3N+6N = 9N                   

2009             2*9N = 18N    9N+18N = 27N             

2011             2*27N = 54N   27N+54N = 81N            

800% increase on N = 8N so total visitors would have been  N + 8N = 9N 

Hence year was 2007.

@200% 1 becomes 3 every 2 years. And:
@800% 1 becomes 9 every n years
The year to year inctrease =3^1/2 =sqrt(3) - 1 x 100 =73.20% annual comp.rate.
9 = 1 x [1 + .732]^n
n =Log(9) / Log(sqrt(3))
n = 4 Years, or 2003 + 4 = 2007.