A hemispherical paper is folder into a hallow right circular cone. The radius of the hemispherical paper is 12cm. What's the cone's base radius, height and volume?

Answer 1

I believe you mean the paper is semicircular, not hemispherical.
The radius of the semicircle, R = 12 cm, becomes the slant height of the cone.
pi*R = cone's base circumference = 2 pi r, where r is the cone base radius.
r = R/2
Cone height = h = sqrt[R^2 - (R/2)^2]

Take it from there.