number_of_rabbits = 2;
print("In generation 1 there are 2 rabbits.");

for i in range (2, 21):

    number_of_rabbits = number_of_rabbits + (number_of_rabbits * 2) - number_of_rabbits;
    # Which is, of course, just number_of_rabbits * 2! 
    print("In generation " + str(i) + " there are " + str(number_of_rabbits) + " rabbits.");