Given population doubles in 20 minutes, what is intrinsic growth rate r?
Attempt: Given population doubles, using exponential growth rate we have $\frac{dN}{dt}=2N$ so $N(t)=N_0e^{2t}$ therefore r=2, but I have a feeling this is wrong since 20 minutes should be used somewhere around here.
Since the population doubles in $20$ minutes, assuming your $t$ variable is in minutes, you should have that:
$$N(20) = 2N(0)$$
If you have an exponential growth rate by assumption, $\frac{dN}{dt} = \lambda N$, which results in $N = N_0e^{\lambda t}$.
If you evaluate this at times $t = 0$ and $t = 20$, you'll find that $N(0) = N_0$ and $N(20) = N_0e^{20\lambda}$. You can then plug in these expressions into the initial equation comparing the population at these two times to solve for $\lambda$, which should not be equal to $2$.