2011 - 1999 =12 years - when the population increased by 1 billion people.
[7 / 6]^(1/12) =1.01292 - 1 x 100 = 1.292% compounded annual growth in population.
Future Population =7 x (1.01292)^n, where n= number of years after 2011
10 = 7 x (1.01292)^n, solve for n
10/7 = (1.01292)^n take the log of both sides
n =Log(10/7) / Log(1.01292)
n = 27.784 = ~ 28 years after 2011 - when the world's population will reach 10 billion people. Or:
2011 + 28 =2039 AD.