Here the growth is exponential. The number of offsprings are increasing in the multiples of 2. 
So the formula used here would be- 
Number of offsprings = 

where n = number of generations 
Number of children in third generation = 
 = 8
 = 8 
Number of children in fifth generation = 
 = 32
 = 32 
This is the most commonly used equation where population doubles itself after every generation.