15.8. Find the smallest positive integer $x$ such that the sum of $x, x+3, x+6$, $x+9$, and $x+12$ is a perfect cube.
15.8. 19
$$
x+(x+3)+(x+6)+(x+9)+(x+12)=5 x+30=5(x+6)
$$

To make up the least possible cube, we must have $x+6=5^{2}$ or $x=19$.