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**viet** (hint: let N be the number of stars in cubic galaxy; $\displaystyle N = 400x10^9$. If each star is separated by a distance D = 5LY, then the total volume of the cubic galaxy is $\displaystyle Vgal= ND^3$. Now the number of civilizations in the galaxy is $\displaystyle n_{civ}$ = 100. Let each one of them be surrounded by an "empty" cube of volume $\displaystyle V_{civ}$ = $\displaystyle x^3$ in which there are no other civilizations--on the avg. So x is the avg distance between these civilization.)

from the hint I got Vgal = $\displaystyle (400x10^9)(5^3)$ = $\displaystyle 2^{13}$.

I'm not sure how to solve the rest.