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**lvleph** Well that equations says the density D is a function of time. More specifically the density varies as $\displaystyle 30\cdot(2^{\frac{t}{2}})$. In the problem it said that 1965 is $\displaystyle t=0$. So $\displaystyle D(0) = 30\cdot(2^{\frac{0}{2}}) = 30$ and we see that the density is 30 for 1965. For 1966 $\displaystyle t=1$ and $\displaystyle D(1) = 30\cdot (2^{\frac{1}{2}}) = 30 \sqrt{2} $, so in 1966 the density is $\displaystyle 30\sqrt{2}$. Do you understand now?