Logistic Growth
1. The problem gives a differential equation describing the rate of change of $N(t)$:
$$\frac{dN}{dt} = N \cdot \left(90000 - \frac{3N}{20000}\right)$$
and an initial condition or value $N = 20000000$ (20 million trucks).
2. This is a logistic-type differential equation where the growth rate depends on $N$ itself.
3. To analyze or solve it, first rewrite the equation:
$$\frac{dN}{dt} = 90000N - \frac{3N^2}{20000}$$
4. Simplify the second term:
$$\frac{3N^2}{20000} = 0.00015 N^2$$
So,
$$\frac{dN}{dt} = 90000 N - 0.00015 N^2$$
5. This equation models growth with a carrying capacity. The carrying capacity $K$ is found by setting $\frac{dN}{dt} = 0$:
$$90000 N - 0.00015 N^2 = 0$$
$$N(90000 - 0.00015 N) = 0$$
So either $N=0$ or
$$90000 - 0.00015 N = 0 \Rightarrow N = \frac{90000}{0.00015} = 600000000$$
6. The carrying capacity is $600000000$ trucks.
7. The initial value $N=20000000$ is below the carrying capacity, so the population will grow towards $600000000$ over time.
8. To solve explicitly, separate variables and integrate, but since the problem does not request explicit solution, this analysis suffices.
Final answer: The carrying capacity of the system is $600000000$ trucks, and the population $N(t)$ grows according to the logistic differential equation given.