A hollow cylindrical pipe is of length 40cm. Its internal and external radii are 4cm and 12cm respectively. It is melted and cast into a solid cylinder of length 20cm. Find the radius(in cm) of the new solid.
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Solution
We have,
l=40cm
r=4cm
R=12cm
We know that the volume of the hollow cylinder
=π(R2−r2)h
So,
=π(122−42)×40
=π(144−16)×40
=π(128)×40cm3
Since, this hollow cylinder pipe is melted and cast into a solid cylinder which length is 20cm and let the radius be r