Size of the population at time t is N(t). About population we know that if its size decreases below the number a, it has no probability of recovering but will decline toward extinction. If it is above another number b, it will decrease at the rate which is proportional to population size. Between a and b, population increases, and there is third number c such that rate of growth increases below c and decreases above c. Draw N'(t) as the function of N(t) for the population.