The formula of a volume os a cylinder:
[tex]V=\pi r^2H[/tex]
r - radius
H - heihgt
The formula of a volume of a cone:
[tex]V=\dfrac{1}{3}\pi r^2H[/tex]
r - radius
H - height
If a cylinder and a cone have the same radius and height then the volume of a cone is three times smaller than a volume of a cylinder.
Therefore
[tex]V_{cone}=\dfrac{1}{3}V_{cylinder}[/tex]
We have
[tex]V_{cylinder}=72\ in^3[/tex]
Substitute:
[tex]V_{cone}=\dfrac{1}{3}\cdot72=24\ in^3[/tex]