check the picture below.
so, a cone, that fits inside the cylinder, will have the same height "h" and radius "r". Thus
[tex]\bf \textit{volume of a cylinder}\\\\
V=\pi r^2 h\qquad \boxed{\pi r^2 h}=15
\\\\\\
\textit{volume of a cone}\\\\
V=\cfrac{\pi r^2 h}{3}\implies V=\cfrac{\boxed{15}}{3}[/tex]
