What length of a solid cylinder 2 cm in diameter must be taken to recast into a hollow cylinder of length 16 cm, external diameter 20 cm and thickness 2.5 mm?
Given,
Length of solid cylinder = L
diameter o cylinder = 2 cm
radius of cylinder = 
Volume of cylinder = πr2L..............(i)
Length of hollow cylinder = 16 cm
External diameter = 20 cm
External radius = 
thickness = 2.5 mm = 0.25 cm
so,
inner radius = 10 - 0.25 = 9.75 cm
Volume = 
From (i) and (ii)
= 
= L =79 cm
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