What length of a solid cylinder …

Given diameter of solid cylinder = 2 cm
Radius of solid cylinder, r = 1 cm
Volume of solid cylinder =  πr2h
= π × 12 × h = πh →  (1)
Length of hollow cylinder, H = 16 cm
External diameter = 20 cm
External radius = 10 cm
Thickness = 2.5 mm = 0.25 cm
Internal radius = 10 – 0.25 = 9.75 cm
Volume of hollow cylinder  = π[10² – (9.75)²] × 16
= 16π × 19.75 × 0.25 →  (2)
From (1) and (2), we get
Πh = 16π × 19.75 × 0.25
  ∴ h = 16 × 19.75 × 0.25 = 79 cm 

Given :  

Let 'H’ be length of a solid cylinder.

Diameter of the solid cylinder, D = 2 cm

Length of the hollow cylinder, h = 16 cm

External diameter of the hollow cylinder, d1  = 20 cm  

Thickness of the hollow cylinder = 2.5 mm = 0.25 cm  

Radius of the Solid cylinder , R = 2/2 = 1 cm

External radius of the hollow cylinder, r1 = 20/2 = 10 cm

Internal radius of the hollow cylinder , r2 = External radius of the hollow cylinder - Thickness  

Internal radius of the hollow cylinder, r2  = 10 - 0.25 = 9.75 cm

Volume of the hollow cylinder  = πh(r1² −r2²)

Since the solid cylinder is recast into a hollow cylinder, so  

Volume of solid cylinder = Volume of hollow cylinder  

πR²H = πh(r1² −r2²)

R²H = 16(10² - 9.75²)

1²H = 16(100 - 95.0625)

H = 16 × 4.9375

H = 79 cm .

Hence, the length of a solid cylinder is 79 cm .