The initial population of a town is 3900, and it grows with a doubling time of 10 years. What will the population be in 8 years. What will the population be in 8 years? What will the population be in 8 years?

This is the problem, I am trying to get the formula for 2 t/T double? The help me solve and example don't show me how to work the problem, it just gives the answer 2 14/8 double ~=~ 2 t/T double, i don't know how the answer is found,