You have to determine constraints!
Current census figures show the world population to be 6,600,000,000 in February of 2008. An earlier census of 1999 had the world population at 5,978,000,000.
If we assume that the population is growing in accordance with the growth model
P(t) = Pinitial X e^(kt)
k = some constant
t = time in years
e= the natural log e
a. Determine the rate of increase, k, and give the exact(as best we can determine) formula for the world population as a function of years since 1999.
b. Now use the 6.6 billion figure as your starting population with the same rate of increase as you determined above and determine how long it will take us to double our world population.