The radius of the base and height of a metallic solid cylinder are r cm and 6 cm respectively. It is melted and recast into a solid cone of the same radius of base. The height of the cone is :

The radius of the base and height of a metallic solid cylinder are r cm and 6 cm respectively. It is melted and recast into a solid cone of the same radius of base. The height of the cone is : Correct Answer 18 cm

Let the height of the cone be h cm
Then,
$$\eqalign{ & \pi \times {r^2} \times 6 = \frac{1}{3} \times \pi \times {r^2} \times h \cr & \Rightarrow h = 18\,cm \cr} $$

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If the given solid metallic right circular cone is melted and recast into a right circular cylinder having the same radius, what would be the height of this cylinder? I. The sum and product of the radius and height of the cone are 31 cm and 147 cm2 respectively. II. Total surface area of the cone is 550 cm2.