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What length of a solid cylinder 2cm in diameter must be taken to recast into a hollow cylinder of length 16cm, external diameter 20cm and thickness 2.5mm?

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Given that diameter of solid cylinder =2cm 

Given that solid cylinder is recast to hollow cylinder

Length of hollow cylinder =16cm

External diameter = 20cm

Thickness = 2.5mm= 0.25cm

Radius of cylinder =1cm

So volume of solid cylinder=π(1)2 h ........(1)

Let length of solid cylinder be h

Thickness = R - r

0.25 = 10 - r

Internal radius = 9.75cm

So volume of hollow cylinder =π16(100 -95.0625)  ......(2)

Volume of solid cylinder is equal to volume of hollow cylinder.

(1) = (2)

Equating equations (1) and (2)

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