The solid cylinder should be to hollow cylinder

Given that, diameter of solid cylinder = 2 cm

Length of hollow cylinder = 16cm

External diameter = 20cm

Thickness = 2.5mm=0.25cm

Volume of solid cylinder = πr²h ------(i)

Radius of the cylinder = 1cm

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

Let length of the solid cylinder be h

Volume of hollow cylinder = πh(R² – r²)

Thickness = R – r

⇒ r = 10 – 0.25

⇒ Internal radius = 9.75cm

So, volume of hollow cylinder = π × 16(100 – 95.0625)

= π × 16 × 4.9375

≈ π × 16 × 4.94 -------(2)

As we know that, volume of both solid and hollow cylinder must be the same, therefore

π h = π × 16 × 4.94

⇒ h ≈ 79cm

∴ Length of the solid cylinder = 79cm